This is good news for lowspec gamers since basically every new steam game may have to be validated for the steam deck, meaning more games should have low end options for weaker hardware. At least thats the hope, we will see how that goes.
I have some doubts that it's ever going to make up enough of the market to cause any significant number of AAA developers to go out of their way to make sure their games run well on the system. Steam certainly isn't going to require games to run on the hardware. And a developer designing their game to impress on the new generation of consoles and higher-end gaming PCs isn't going to limit it in any way to support a device that only a fraction of a percent of its potential customer base will have.
Realistically, I don't see the device selling much more than a few million units, if even that, while the consoles from Sony and Microsoft combined tend to sell in the vicinity of 170 million units each generation. If any gaming hardware encouraged developers to support lower-end hardware, it would be the Nintendo Switch (or its eventual successor), which has moved over 100 million units over the last five years, though there's no guarantee that the PC port of a game will see the same level of optimization. The same goes for the previous generation of consoles, which many games are still designed to support, though that will be less the case as the install-base of new consoles grows. Games built specifically for the new consoles will likely be a lot less friendly to the Steam Deck than the multi-generational titles that make up most of the current market.
Phones are another potential market that developers may want to target, and there are some examples of good games designed for mobile that also see decent PC ports. Again though, that's a far larger market than the Steam Deck will ever be, so I don't think the Steam Deck itself is likely have that much of an effect on support for lower-end hardware.