Seem to be a case-by-case basis on both sides. What would prevent games with an unlocked frame rate engine on ps4 from going120 fps without any patch in boost mode?
Nothing, but truly unlocked multiplat games are extremely rare. Most games that let you 'unlock' the framerate are really replacing a 30 fps cap with a 60 fps one.
The only one I know of is Rainbow Six, but that's getting next gen enhancements anyway.
A game with no frame cap would start tearing when it went above 60 fps. Almost nothing works that way.
Developers have two options on both PS5 and Series X. Firstly you can, patch the old version of the game to allow it to target higher framerates as games like Hitman 2 allow, or to add modes that target 4K native, or similar, but still be limited to GCN features and not leverage the new storage sdk. Secondly they can make a dedicated SKU for next gen that fully leverages the new hardware. Nothing prevents devs doing this on either next gen system.
On Series S/X there are two further possibilities. The dev can update the game to use Smart Delivery and have the one SKU fully leverage RDNA2 and the full storage improvements.
Or, the back compact team can work their magic optimizing games on a title by title basis, increasing resolution and framerate without touching the original code.
You can reduce that to say 'either way someone needs to improve the game, so what's the difference?' but it ignores that there are benefits to what Microsoft are doing and that it will lead to games getting updated in some form that otherwise wouldn't be. For example, MS have updated games that have been delisted, and games from developers that aren't around anymore, or that aren't in any position to work on the old code.
Yes all the above require work. Yes MS won't be doubling the framerate of many games. But only a few developers are going to patch games more than a year old even if they remain in a position to do so.
I don't expect old games to get updates so I'm not frowning on Sony's approach here. I'm really happy all my Playstation games are coming along with me. I'm happy my PSVR is going to work etc. What MS do is essentially add frosting to a limited set of games that wouldn't otherwise get it. I really appreciate that extra, but it's still something I see as a neat bonus.
And I know people pointed to PSVR to invalidate Grubbs point about the PS4 not being able to output at 120hz, but what he said is true for TVs even if he overlooked PSVR. I can hook my One X up to my TV and output at 1440p and 120hz. I can't do that with my PS4 Pro. It doesn't support it.