If the machine is indeed fully BC with my PS4 library, then I literally have no reason to keep my PS4 Pro. So I'll transfer whatever digital games are on the internal HDD to my external SDD, wipe the machine, get it nice and clean, then fucking sell it. That alone would make up for any additional expenses I may have on the PS5. With the exception of my backwards compatible PS3, I usually sell or gift my previous consoles. I generally had no need for the older machines once the new one is released, granted I can play my old library on the new machine.
I'm very, very curious about the backwards compatibility of the PS5. I have a massive library of games (many of which I've literally never booted up once) that I'd love to install on my PS5, and get to them eventually. I really don't want to have an extra console hooked up to the TV if I don't have to. I already have my PS4 Pro, Switch hooked up to the tv, and occasionally the PS3 if I really want to play a game I can't play on PS Now (of which there are many sadly).