But… you don’t really have to use Gmail to use most of Google’s other products, just as you don’t really have to use (actively) Google+ to use other Google products either.

Unfortunately no. You are going to be tracked whereever you go and it has GA. The reason why Google can stop reading your emails is because they have a global network now that can track you better.

But imagine you use an Android phone, for instance. There's a whole bunch of integrated stuff that works well together that wouldn't if you had a Google account and then kept your e-mail somewhere else. It's not really a 1:1 comparison.