>Could somebody point me to a technical explanation of why it's sometimes non trivial to just compile your app against x86-64 and call it a day?

https://docs.microsoft.com/en-us/cpp/build/common-visual-cpp...

http://www.informit.com/articles/printerfriendly/2339636

BTW: The 32bit limit is per-process, not for the OS. You can easily have multiple 32 bit processes consuming more than 4GB memory in total. You don't need to port to 64bit to get that extra memory.