IMHO this is kind of silly to use very old versions of gcc, libc, etc. Instead use the kernel's namespacing, chroot, etc. features to ship your dependencies. I.e. build a flatpak and forget all these archaic tricks and machinations.
Even worse, while there is an option to install things in the user's directory only (so i can make a separate user to try some things that i can easily delete later) not everything installs with that options and wants root access to pollute the rest of my system.
Nowadays i simply avoid anything related to flatpak. If something doesn't provide normal binaries and i really want it, i'd rather compile it from source (and if the source language is something exotic or the program needs a ton of dependencies i'd just skip it).
Practically most container images literally are their own distro install. In the case of flatpak, this becomes obvious when looking at flathub and the provided "runtimes". At that point they just reinvented packages.
If Flatpak checked to see what i have already available and downloaded only the dependencies i do not have already installed while also providing seamless integration (a large part of which wouldn't be an issue if it used the system libraries/software), it'd actually be very useful and a solution to every distro having to package every program under the sun and keep it up to date (which IMO is also a bad thing because obviously not all distros can do that, it is just that Flatpak is a worse solution).
On the "unnecessary stuff" side, do note that Flatpak uses a diff'ing algorithm for the runtimes such that if you install several, they won't take up duplicate space. This is a a reason I prefer them to AppImage.
Closed source? Your users are paying you for the headache of making your software run wherever the users need it to run. The differences aren't really that big unless you're shipping something deep in the stack or have to support ancient systems. And you still have to test software everywhere.
That's great when the distro maintainers are packaging the software I want—and a problem when they aren't!
> Closed source? Your users are paying you for the headache of making your software run wherever the users need it to run.
"Okay, then I'll ignore Linux because the users who need it there are too much effort to support."
The core of the issue really - and Flatpak is just an attempt to work around it, not solve it, but just work around it - is that unlike the kernel the majority of foundational libraries do not provide stable ABIs.
As i wrote in another comment in this post, you technically can run software from the late 90s as long as that software is either statically linked or dynamically links only to the GNU C library and X11 (for GUI stuff), since the kernel and those two have stable ABIs since the 90s. However you do not get much functionality from those libraries even for a notepad-like application and anything beyond that tends to not provide stable ABIs.
And that is a cultural problem, not a technical one as there are several examples (including Windows as one of the most obvious) showing that you can have backwards compatibility and stable ABIs while also being able to extend the functionality of the underlying libraries (e.g. an app i wrote and compiled in WinVista got support for an emoji input box automatically in Windows 10 even though i never did anything for that beyond just using the functionality for text input that Windows provided since the late 80s).
Because of this the "(A)" solution you mentioned wouldn't even work since, say that everyone agreed to use -for example- GNOME. But that would only work until the Gtk developers decided to break Gtk's ABI yet again for their newest version, forcing everyone that decided to depend on them to waste time "upgrading" (the issues of which can be seen with XFCE or even better, Gimp that still depends on Gtk2 until at some point in the future when the Gtk3 is done - an move that a Gtk developer once described as upgrading from an abandoned version to a deprecated version).
I'm not singling out GNOME/Gtk though, the same issues apply with KDE/Qt since Qt also breaks their ABI every major version. If anything KDE/Qt are at a worse position by relying on C++ since despite how many promises GCC developers might make, with the C++ standard not providing a stable ABI promise, there is always the sword of ABI breakage looming over the head of any program that rely on dynamic libraries written in C++ (or any other language that doesn't have a stable ABI). Technically this could be solved by providing proxy dynamic libraries that use the old ABIs to forward calls to the new libraries but i am not aware of this being done in practice (i think Haiku does this to support BeOS binaries compiled with gcc2 but i'm not sure). I do not expect any C++ library to do that on Linux though.
So that is the root cause. It requires a cultural shift from often not even acknowledging the problem exists to actually considering ABI breakage something to avoided by all foundational libraries.
Flatpak is just a workaround that relies on the kernel at least not breaking its ABI, but even as a workaround it isn't that good for the reasons i mentioned. As i wrote in another reply, it could be improved - as a workaround, not a real solution - by only installing the libraries and software that doesn't already exist in the system and having better integration with it (with a large part of the latter being solved by doing the former). Fortunately so far most libraries do not break their ABIs every year, so at least chances are when i decide to download Notepadqq, it will use the Qt5 installation i already have in my OS instead of downloading its own copy.
It sounds like you probably want systemd portable services: https://systemd.io/PORTABLE_SERVICES/
I don't think Flatpak is the right tool for the job, it's clearly optimized for UI tools and the occasional command line invocation. Snap may be better for composing a working systems, though that comes with the obvious Snap downsides. Systemd has some great tools to accomplish image/container based operating systems, though I'm not sure how common it's used.
If they don't want the container, they can typically compile the source too.
I have one server with a complex firewall configuration that simply won't work well when I install Docker on it. I end up finding a lot of alternatives for what would otherwise be obvious solutions because projects stopped shipping binaries and replaced them with Docker containers.
I'll gladly make use of the compilation approach thanks to software available in the AUR. The whole "let's ship a copy of Debian to make my 400kB binary run" approach is as tiresome as "let's ship a copy of Chromium so I don't need to learnddesktop UI".
I know how terrible the dependency management situation on Linux is (I myself have had to resort to building software in chroots because of glibc being outdated) but the "modern" approach seems to add more trouble than it solves when I try to run programs like this.
On many distros these problems have luckily already been solved. On Ubuntu that means I'll often be using an outdated version of the program that works just as well and on Arch (based distros) that means waiting for a compile every other update, but I've given up on tools that come with an entire ecosystem.
AppImage does provide that "download an exe and double click" experience but it assumes you have made said portable Linux binaries (and they still have the drawback of dragging in a bunch of libraries you most likely already have in your distro).
Why not do it in reverse? Offer binaries for the people running regular systems, and then let people with irregular configurations compile or containerize the program themselves.
Seems like you'd save on a lot of redundant packaging and wasted data that way.
Does dl_open crash if it can't find an open ALSA connection or something?
dlopen("plugin.so"); /* Load that super cool reverb plugin */
....
/* oops, the .so is not part of the flatpak ... fail */
Static complied dependencies goes a long way.
If that sounds silly... then ship your libc and everything you depend on in a flatpak or similar contained system image.