A lot of modern games recognize that some people have poor eyesight and offer UI scaling options. Especially the games with lots of text to read. In others, the text is a fixed pixel size and gets proportionally larger as the resolution decreases.I think the screen will be a much larger barrier than the hardware itself.
A lot of PC games are released under the assumption that most players will be using a mid-to-large monitor, and even on "13-"15 laptops they can be a pain to play. The Deck is half as much, with a "7 screen. You can adapt most games to be enjoyable on smaller screens, but most pc games are not by default, and they won't be patched specifically for the Deck. For example, the majority of Strategy games have small text boxes and numbers integrated in the interface, with no options to resize them, and they will require either you to have exceptional eyesight or be an absolute eye-straining misery to play.
As someone who frequently tries to make games run at 540p, I'm fairly certain the Deck's screen size will serve just fine.