Re security: This is probably best fixed by packaging apps with SELinux rules, which are enforced and cannot be disabled, plus responsive maintainers. There could be more than one app version to allow different SELinux rule strengths, per-app.

Re disk space: no mention of Nix? That also solves updateability for security issues. Another solution is Gentoo's -- deliver the sources and recipes, (re)build as needed. That could also work in userspace.

It would be nice to be able to 1) distribute Linux GUIs 2) let friends install them easily 3) have the friend edit the GUI app logic and run the modified version. AFAIK, this is currently only possible with apps based on interpreted languages (e.g., Python+GTK), but even there you'd need to guide your friend to install the required packages.

Re "the state of software complexity in 2021": Ignore complex software, and just don't include it on your system. Start with a subset, and add only what you need. That's how OpenBSD remains manageable. If people want to extend, they can go ahead. But you don't have to fix the world.