I would personally have someone at Valve sit down and ask the developer if the game they sent in is a prototype or the game they want to submit. If the answer is “What's a prototype?” the developer goes bye-bye. If the answer is “It's a prototype” or “It's the finished game,” that person at Valve would then play it, and then, up to three scenarios happen, depending on what the answer to the question was.
If it was a prototype that's fully playable and launches on Valve's computers fine, Valve sends it back and asks them to put in some extra polish and release it in Early Access. If it's a prototype that seems like it's had work barely started on it (like After Reset RPG, for instance), Valve sends it back and asks them to submit it again for free after a lot more work is done. If the developer refuses to put in more polish or won't work on a “barely started work” further, the developer goes bye-bye.
If it's a finished game, and it actually works the way it should, it goes on to the store front. If it's a finished game with some key features missing, big bugs to iron out or assets clearly meant as placeholders, the game will go into Early Access. If the game is clearly in a state that needs more work, or, despite the developer's assertions, is still in the prototype stages, Valve then asks them to give the game more polish. If the developer refuses to give the latter game any polish, that developer (you guessed it) goes bye-bye.
Making a prototype of a game is a big part of game development, and Valve, of all businesses, should know that.