Although I understand your worry, decision by committee will never be a good way to move things forward, as there will always be disagreements. To be able to move forward we have project leads that make the final call on things. We believe Gutenberg will be in a good place by the time we decide it’s ready to release WordPress 5.0 (you’ll note there’s no release date set, that’s because we are intentionally leaving it up to the Gutenberg project to be ready before we plan any release around it, thus giving us time to weed out the bugs and make it the best possible version of it self it can be).
Most plugins will not be affected by Gutenberg, only those that interact with the editor directly (such as page builders in some cases). WP Bakery have already announced they’ll be compatible, as have the other builders I’ve looked into so far.
As for themes, nothing should change for them, they should work out of the box, the only thing is that they may not have all the fancy new possible styles (that make things look shinier, but are not required). If a theme breaks when WordPress 5.0 comes out, I can with an almost 100% certainty tell you it would have broken regardless (possibly not the most optimistic answer, I know, but I see no reason to sugarcoat that bit). We are working with backwards compatibility in mind though, so we do our best to fall back to the classic editor if something is detected as likely to break.
Now, where things -might- behave poorly is with custom work and specially crafted websites, and for them we’ve created the Classic Editor plugin that disables Gutenberg until they’re ready to use the new editor (and we are making it very obvious how to do so with the callout, informing of both options with it to help educate everyone).
I do hope that helps a little, even if not everything is super optimistic there are plans for the scenarios at the very least and I don’t think we’ll see too many problems when the new editing experience is ready for launch.
