If Steam Deck is a success then I expect knock-off Steam Decks with such alternatives would be available in the near future.

It's arch based, you can probably make it cooler, less powerful, and turn the fans off.

Given the heat output I'd not count on a cool fanless version showing up anytime soon. Not without significantly less performance

Couldn't you just underclock and undervolt the current day SteamDeck to achieve that?

Or in general set a really strict power envelope for the AMD APU used in it?

Have others found issues with the heat? From the reviews I've seen they kept it away from the user's hands.