I'd say there's a reason PC gaming handhelds popped up when they did and a large part of it is that APUs has reached a level of performance per watt where they actually work, I.e. provide decent frame rate in popular new titles (in 720p). Putting in an older part will give worse performance at the same lousy battery life and you can't really drop below 720p without getting compatibility problems.
And if anyone wants to say "but the Nintendo Switch is running on super old hardware?" then please keep in mind that there is a world of difference between consoles and PC even with "standard" hardware in them (x86 or ARM). The fact that all Switch games will run on the exact same hardware opens up for a level of optimization that just isn't realistic on PC and that extremely diverse landscape.
What could, but likely won't, happen in the future is standardization around one APU per two/three years such that all gaming PC handhelds use the same APU and then differentiate on other parts, like Legion GO vs ROG Ally. Then it becomes feasible for developers to do targeted optimization to that APU.