technically Series S could even do 4k60fps. It all depends on the game, some others even 900p30fps may be to much.
The PS3 can do 1080p/60fps, the Switch can do 1080p/60fps but Breath of the Wild runs at 900p 30fps and plenty run at 720p 30fps.
What a system can do is all relative to the demands of the game. Just as we've seen this generation, most single player games are designed around 30fps and maximum visual fidelity. This is unlikely to change. Internally Microsoft teams may target 60fps but we know thats not the case for a lot of 3rd parties.
Ok, so this is getting to crux of what I'm asking. I was under the impression that the Series S is powerful enough to give the best possible performance at 1080p, and that the extra TFLOPS, RAM, processing speed, etc of the Series X is wasted in 1080p. So if a game is designed to run at 900p/30 it can potentially run at 1080p/60 on the Series X? Is that possible? I thought if a game was designed for a specific resolution/fps it can't go any faster no matter how powerful the console.
Come on, man. You really think that if it says 1080p/60fps on the box, then it will run _ANY_ game, no matter the graphics, polycount, use of raytracing, usage of machine learning at 60fps? Like was noted, the PS3 also had 1080p/60fps on the box.
Games are designed, tuned and tested for each specific console. And it's up to the developer to choose the tradeoffs in resolution, framerate, polycount, etc. One developer may choose to go 1080p/30fps on one console and 1080p/60fps on another. Or 1080p/60fps on one and 4k/60fps on another. Or some combination of the two with upscaling/checkerboarding/something-like-DLSS/dynamic resolution. It's a choice, and something that you have to build in, tune, and test for each game.
And to your question: some games are locked to 30fps by design. And some games can scale easily from 30fps to 60fps, 120fps. It depends on how the game engine is designed. Some games, especially older ones, had a fixed clock tick set to 33 ms (30 fps), and a lot of calculations were done based on this. In other words, if you ran a game like that at 60fps you could get weird physics, or collision detection that's off, all kinds of stuff. Games based on modern engines like UE are usually not like this, though.
Nobody knows how games will run on the XSS right now. It's likely that games will be designed to run on XSX/PS5 because they have comparable specs, and then each developer will make a choice on how they think their game presents itself best on the XSS. That might just be limited to a resolution drop, but it could entail other things. Keep in mind that the XSS has less memory than the XSX. So it's not just a straight forward thing to run an XSX game on the XSS. You need to do some work to make it happen.