I think an RFC period is a great idea, especially for major changes such as this Customizer one. While I can certainly understand the need for standards and why the decision was made, I don't agree with the decision to make the Customizer mandatory. The current incarnation of the Customizer is a horrible user experience.

Whilst I try my best to keep up with discussions in Slack along with emails from the various Make WordPress teams, it's not always possible to know of every single change that is being discussed. Like others here, I already contribute way more than 5% of my time to the WordPress project, from organising monthly Meetups & WordCamps through to supporting themes & plugins. And I'm happy to do this, as I love the community, but again, being across everything is impossible, especially when you still need to do client work to pay the bills.

I think some advanced notification of major changes, even if it was a blog post on the .org site (and I'm talking about the 'News' section, not tucked away in one of the many Make WordPress sections) would be great. News items there certainly get more exposure than the updates in the individual Make groups.