All my 32-bit plugins broke in Ableton Live, and Proton stopped working when I tried Catalina. Without my music toolkit, games or preferred programming environment, MacOS doesn't really have much left for me these days.
But essentially storage and memory savings, maintainability savings and some issues with the ObjC runtime.
I assume part of the issue too is being able to enable pointer authentication, which afaik uses the higher end bits to store data, and being able to do that allows them to secure the OS better. So 32-bit support likely was a security risk factor as a result too.