If you don't want new features, don't upgrade it, what in the non-sequitur is this? I get the argument that it SHOULD be OPT-IN rather than OPT-OUT, but that would require annoying pop-ups every upgrade that explains the new features and ask if you want to OPT-IN. That is more burden on the developers and will annoy more users than benefit.

If you are concerned, they do have what is called a 'changelog' that will explain all of the new features and how to switch them off if you like.