On my desktop there might be a few apps that don't need to upgrade but on my phone, 90% of all apps are tied to a service. Uber/Lift, bank apps, apartment apps, hotel apps, bike rental apps, social media apps, dating apps, streaming music apps, video apps, etc.
Yes because for example I'm sticking to K9 mail 5.6 because of the UI change in the next versions. I picked K9 exactly because of the original UI so I'm not using the new one, which is like every other mail client. I saved the APK and I'm installing on any device of mine, also when my fingers slip on the update button. Uninstall and install 5.6 from the file manager.
And no, because if the company behind the app wants to force people to upgrade they'll make the backend incompatible with the old app. Also no, because I don't think that many normal people invest time not upgrading an app because of the UI.