Why their devs choose to upgrade toolkits this way I cannot say.

I forget the exact details but iirc Evince was a special case because rendering PDFs in GTK4 was so different that they essentially just had to rewrite the whole application. I think Gnome Papers still doesn’t support the full feature set that Evince supported (although it works well for most use cases now). This is why its still not the default for Gnome, although I think Ubuntu has decided to adopt it a little early.

I’m lucky in my job that most of my work can be done on Linux with FOSS software. But I would use Windows if this weren’t the case, and I know for some colleagues who need to work with specific proprietary software, there aren’t that many other options available, and its far easier for them to just use Windows (or perhaps macOS if they have the option).