Valve, a AAA developer in their own right, already has plans to release their own hardware & doesn't need to wait for a third party to make hardware for this to succeed. They already have a massive base of customers which have already invested hundreds, if not thousands of dollars, in their Steam libraries so they don't need to build traction. Game consoles, while a significant expense, are not an all or nothing commitment like a cellphone - customers can easily have a Steam box in addition to their PlayStation, Xbox or PC.

It's not really fair to compare this to Android. Android succeeded, not because of Google's user base but because of Google's financial clout. Companies thought that throwing in their lot with Google was a safe bet - there's too much money behind the project for it to fail. People are going to throw in with Valve because it's a smart bet - it's bringing what gamers want to the table.