I agree that we shouldn't worry (at least for the moment), but I think the main reason is the lack of locks, both when it comes to hardware (no locked bootloader) and software (getting root access is trivial, so you can uninstall whatever components you might not like and with updates not being mandatory you can keep it under your control).

With SteamOS, you already have an ecosystem, which is Steam. There is (at least for now) a clear distinction between Hardware manufacturer and software provider.

Currently, the only officially sanctioned version of SteamOS is the one that is shipped with Steam Deck (even though that might change soon), which is hardware sold by Valve (ie, the same company making the software). Meanwhile, most people using Android don't use Pixel / Nexus devices and thus their hardware is not being sold by Google.

So I'd say this depends entirely on how do the new manufacturers wanna go about it when it comes to offering their own custom versions of SteamOS. At the moment this is ok because Valve has been acting as a "benevolent dictator" and they have essentially had a monopoly on SteamOS 3 devices until now. Once that monopoly breaks (and if Valve actually allows third parties to ship their own customizations) we'll have to see what kind of control will their partners want to assert over it.