If I can have a dockable PC that can churn decent frame rates, It's just a matter of switching the input on my monitor and switching the bluetooth mouse/keyboard to it. For me, the fact it's portable is just icing on the cake to be honest. I'd buy it even if it didn't have a screen or controls to be honest.
I have a Steam library full of games bought on sale that my Mac was never quite up to the task of running, and the prospect of having them all on a neat little handheld for less than the cost of a midrange iPad is awfully compelling.
That said for playing 2d or lighter indie games it will probably run fine at native 1440p... assuming with external display that you can use native res.
Coming from previously having a top-of-the-line ($6k+) gaming PC a decade ago that still does "okay" on most titles at 1080P to trying to game via GeForce Now from my Macbook was a shockingly bad experience. I'm very interested in the SteamDeck specifically because it allows me to use local hardware to run the game without requiring me to build another $6k+ gaming PC which I might get to use one day a month.
Alternatively, gaming laptop. I’m missing the use case of the Steam Deck except in the event of a budget crunch, since you can get ultraportable 14” gaming laptops that have good oompf - examples being the Asus G14 and the Razer Blade 14.