Miners can't just go and change the protocol. They risk that non-miners won't accept new blocks, so if they are going to enforce different rules, they need to make sure that everyone (not only other miners) will be okay with that.

Of course, some people can upgrade blindly. But if in the middle of transition it turns out that some people are worse off, everyone will rush back to avoid wasting resources on blocks that are not 100% accepted.

Well, you don't regularly see contentious protocol changes. I agree that miners are mostly following the technical leadership, I'm not sure it is 100 years true.