What will happen to minecraft after windows 10?

It will go back to the 90s where it belongs

Microsoft will continue to put it on every device known to man, and will ruin it by adding more paid add ons. Eventually the original PC version might be abandoned, and they'll just focus on the inferior "Bedrock Edition" of the game.

Win 10 is going to be around for a long time. Microsoft announced when it first came out that it was the last iteration of Windows that they would do. The idea is to keep the name and do updates to it several times a year. So don't look for a new operating system from Microsoft anytime soon.