> Why do we even have linear physical and virtual addresses in the first place, when pretty much everything today is object-oriented?

Maybe it's because even though x86-64 is a 64-bit instruction set, all the CALL and JMP instructions still only support relative 8-bit or 32-bit offsets.

> Translating from linear virtual addresses to linear physical addresses is slow and complicated, because 64-bit can address a lot of memory.

Sure but spend some time thinking about how GOT and PLT aren't great solutions and can easily introduce their own set of security complications due to the above limitations.