It's not about stopping the evolution of web technologies, as you say, it's about guaranteeing a minimum baseline of features that we can be sure is going to be present. A minimum target that you can count on. Most sites have no need for the advanced features you mention, but if they do then there's nothing to stop them requiring that support, and browser manufacturers will certainly keep rolling out new features as they fight for market share. In the end, it's about moving the baseline forward.