There are plenty of supposed Steam Deck killers available in the portable gaming space, from the Asus ROG Ally to the Lenovo Legion Go and all the Ayaneo and ONEXPLAYER devices in between. But none of them have managed to outright slay the dragon that is the mighty Steam Deck yet.
This website provides a better explanation and use cases than anything I could write. Some of the highlights:
Interestingly, Gamescope also provides a way to independently set max frame rate for the game when it is focused and unfocused, you could set it up to something really low when unfocused. Also interesting is the upscale options, you could use integer scaling for those old games, or force FSR on any title (although results can be mixed because the game UI will also be upscaled).
Gamescope becomes a very interesting option when you use it on a machine that doesn’t have easy access to a keyboard and mouse, like a handheld, a “consolized” PC or even a “normal” PC that double duties as a “console” (playing games on a couch, despite using a desk for normal usage)
Like, I remember a friend of mine saying he had trouble running Sonic Generations on Windows because depending on what he was doing, he was either playing it on a monitor or on a TV. The Game for some reason detects that change and throws a fit, asking the user to reconfigure its graphical settings. Gamescope can lie to the game and force the game to see an arbitrary resolution.