Sony have profited a lot from cross-gen sales for games that first were just PS3 launch. So I really expect some of the best seller will receive a vamp-up for PS5 and get additional sales even if BC exists. Now, only patch already sold games that person won't need to buy another game I don't think it will happen. At most patch to 60fps.
Yeah I'll probably not buy any ps4 games at launch if they release within 6 months of ps5 so I'll play the upgraded ps5 version.