When Gutenberg becomes part of core, existing content won’t break, because it’s merely an editing screen, it does not impact existing content.
There are also safeguards that will be in place at that time, that are not there now because this is all used for testing and we need to know about any potential failures so taht we can pick up on them early and look at ways to work with them.
This topic is spiraling a bit though. This is a review, not a discussion topic for everyone to jump into, please feel free to open a forum topic for that. In light of that, I’ll be closing this off to new replies at this time.