First thing, I absolutely hate cross gen. But it does raises some question regarding cross save and upgrade cost. I do think Sony not gonna do either but there are some example that may force Sony's hand.
Cross save - Insomniac said no you cannot transfer save for Spiderman but has since backtracked and allowed the bit cumbersome cross save method where u need to have the PS4 version, upload save to cloud from in game then open the PS5 ver, download the save. This on top of many other PS5 upgrades like Nioh, Sackboy, Call of Duty and Borderlands 3. I think sony devs will implement this features easily.
Price and Upgrade Cost - $60 vs $70. Free PS5 upgrade vs no PS5 upgrade vs paid PS5 upgrade. So far Sony has taken the stupidiest forms which is no PS5 upgrade at all, requiring purchasers to repurchase if they want to experience next gen ver. So many 3rd party publishers have allowed either free upgrade or a $10 ($30 for EA Battlefield lmao) for many titles which may forces Sony to concede and make their exclusives have an upgrade path. I hope its free cause Oh My Gosh Sony you already developing a separate PS5 version just let users upgrade for free when they get their PS5! Xbox also give free upgrade paths to their games like Ori, maybe a bit of competition (if Xbox ever discover how to publish games again) can give Sony the incentive.