That's insane but you're right. Firefox on Ubuntu had awful trackpad scrolling 10 years ago and it's still bad. How do you make an OS where the main pointing device on half of the market sucks and assign that low priority?
Because Linux isn't "an OS". It's a kernel made by one set of developers, combined with a bunch of operating systems made by a second set of developers, which pick and choose compositors/window managers/etc. often made by a third set of developers. Each of these sets of developers are pretty good at solving bugs that live entirely in their "domain", but when there's an issue which crosses these interface boundaries, there is nobody to "assign priority", never mind actually work to fix it.
(Not to mention, systemd demonstrates that trying to solve these kinds of pan-system problems earns you little gratitude but tons of vociferous hatred, so people are not inclined to do it.)
Anyway, it's nice to have different settings that suit different people.
1) It's too fast. My Android phone also scrolls like XINPUT2 but it's got a smaller acceleration. But maybe it's what you say about getting used to it.
2) That would work in Firefox and every other window would still scroll in the other way. That's really bad and works against getting used to a specific scrolling behavior. I don't get it: the scrolling behavior should be managed at a level below every application. All the scrollbars should behave in only one way, except the random application that implements its own logic because of some important reason. So I would expect a system wide setting for XINPUT2 or traditional scrolling.
If you want it to not scroll further you need to stop scrolling before lifting you fingers. That should be pretty easy to get used to.
The interesting thing: Some Distributions have smooth scrolling (or interial scrolling / kinetic scrolling) by default (Fedora, Ubuntu) and some don't. Those who have enabled it, have no speed setting, so most of the time it's way to fast. I tinkered around a very long time with libinput-config to get it right and now it's acceptable. But it is still waaay better on macOS.
It is NOT a hardware issue though. I tried "hackintosh" on my T460s, and the touchpad experience is nearly as good as on a MacBook, so it is mostly software / OS.
Please don't perpetuate the awful terminological confusion around this issue.
I also think that the App developers should not have to shoulder this. App developers have way better things to do than care about touchpad events... like fixing issues and developing new features. I don't see where smooth scrolling should be an App feature...
And Qt apps will scroll differently than the GNOME project's apps which will scroll differently than other Gtk apps. It really feels disorganized and unfinished.