But that means I have to update my OS and my application when I want a new version instead of just my application. Say what you want about Windows, but I can download and run the latest version of all my applications on Windows 7 without it missing a beat.

Yes, because either you, the user, must track down .NET Framework X.Y (less of a problem now that they're stabilized) or because the application packs all of it dependencies and shoves them into \Program Files or \Windows\System32 until it's eventually 30gb with 1000 copies of msvcpXXX.dll

That's not that different from the distribution model of Flatpaks/AppImage (or APK, or .app on MacOS/iOS). It's more that, traditionally, Linux packaging solutions try to only have ONE copy of glibc or any other library, and packages are recompiled from source so the symbols resolve. Something which isn't an option on Windows, as a generality.

I don't think Windows is a good comparison, since major upgrades for it tend to be vastly more invasive than major upgrading the average Linux distribution. Debian 6→7, ten years ago, was a really massive infrastructure upgrade, but ever since it's been pretty smooth sailing. Ubuntu is a bit bumpier, but it's still only on the level of Windows' "major feature upgrades (that totally aren't service packs because we don't want to extend your warranty)" updates that Windows 10 and newer get every 6 months.

And Windows 7 is extremely old and only works with new software because, and as long as, developers go the extra mile to make their software work with its old APIs. Valve recently announced that they'll drop support for it next year, and other companies will follow soon. It's not too much different from the situation of, say, Debian Oldstable or Ubuntu LTS: Outdated, but popular enough that people tend to put in the effort anyway.

Hackers can also run their applications on your Windows 7 installations without missing a beat

Is that true?