Yes, Flatpak sounds like it solves the compatibility problem (although it remains to be seen in practice what issues itself might have) but it still is half the solution: a Gtk1 could exist, but since that Gtk1 runtime is still the abandoned Gtk1 libraries (which were abandoned because Gtk2, Gtk3 and now Gtk4 decided to break backwards compatibility), it doesn't provide any new functionality and/or fixes to programs linked against it. In contrast in Windows, since Win32 is both backwards compatible and continues to get developed, if for example i use a program from Windows 95 that uses the common file dialog, i still get to see the overlay icons from my VCS extension or the compressed file status. On the other hand Gtk1 would still show the old Motif-like open dialog that was written back in the late 90s.
Or perhaps more fitting to modern Linux desktop, Gtk1 and Gtk2 will not support Wayland nor any new functionality introduced for scaling (did you know that, despite common narrative from Wayland fanboys, X11 already provides the necessary functionality to implement per-monitor and even per-window scaling with pretty much 99% of the same code that a toolkit would need for Windows 8+ scaling, that is most likely already written? It only needs some cooperation from the window manager and this sort of code could even be implemented for XWayland.... but it needs a bit of modification from the toolkit's side too and while it could be done for Qt5 and perhaps Gtk3, it'd need Gtk2 and especially Gtk1 to be worked on, which wont happen - on the other hand, if Gtk2/3/4 were backwards compatible, all programs would benefit from such new functionality).
Or perhaps more fitting to modern Linux desktop, Gtk1 and Gtk2 will not support Wayland nor any new functionality introduced for scaling (did you know that, despite common narrative from Wayland fanboys, X11 already provides the necessary functionality to implement per-monitor and even per-window scaling with pretty much 99% of the same code that a toolkit would need for Windows 8+ scaling, that is most likely already written? It only needs some cooperation from the window manager and this sort of code could even be implemented for XWayland.... but it needs a bit of modification from the toolkit's side too and while it could be done for Qt5 and perhaps Gtk3, it'd need Gtk2 and especially Gtk1 to be worked on, which wont happen - on the other hand, if Gtk2/3/4 were backwards compatible, all programs would benefit from such new functionality).