If TLOU2 comes out before the PS5, than expect a TLOU2 Remaster for the PS5 and not being full BC. I cannot see Sony dropping out from Remastering games since they made a crap ton of money and sold a crap ton of copies using this method. Its probably better this game becomes a shared PS4/PS5 game similar to Zelda BOTW in my opinion.
This is how I see things playing out. Same thing with original TLOU.