Valve makes a lot of money from the steam store. They’re not going anywhere. There are competitors like gog games that sell them without DRM. You download the games anyways so I’m sure a solution will be figured out if Valve starts to have issues.
Steam provides a pretty seamless experience for gaming, and it provides useful services to developers as well. Then you have things like the steam workshop and marketplace.
And for minecraft being open source: who cares? Gamers want games that are good and fun to play. There are very few open source games that are actually fun to play.