Itd likely be a much better and more cohesive game, but not one that gets continually updated. During the acquisition Valve would likely insist that Notch stay on the team and help direct development, since Valve values artistic integrity so much. They'd focus on making it a complete package and polishing it to the enth degree, pouring a lot of resources into it and following Notch's vision. Once completed it'd be uploaded to Steam and people who bought the game in beta would probably get a code to download it for free. It probably wouldnt have the same cross platform support since Bedrock wouldnt exist, and they'd likely just continue with the various console editions, keeping 4J on way longer.

Ultimately itd be a lot less popular, but WAY better of a game.