I might give you Windows 7 on functionality, it has been forever since I used either. But definitely not design. 2000 has a UI that is consistent throughout, clear, and professional. It's a masterclass in UI usability engineering. Plus it's also heavily customizable if you want to do so. A lot of that was lost with Vista and some with XP.

AppImages are precompiled archives with extra steps. Meh. No, some of my problems with Flatpak are:

• it conflates app sandboxing with app distribution
• it mandates using bespoke APIs to work in sandbox mode instead of the established APIs (to the point where I've heard "we can't implement X, it needs to work in Flatpak")
• these APIs are often very Flatpak-focused but regardless become the standard for non-Flatpak because there is no existing alternative
• it ships its own builds of code that should be part of the system (for example, UI toolkits which would otherwise load global plugins, breaking stuff such as IME or themes)

Some of that (and why it's necessary in the first place) is due to Linux's incredible fragmentation and lack of an extensive backwards-compatible system API (such as macOS's Cocoa), which causes a lot of other problems everywhere – but a lot of it is also self-inflicted. In fact, the massive focus on Flatpak and looking like that is the direction the Linux desktop is going was partly what drove me to try out a Mac.