Shipping the Template Editor as opt-out with 5.8 is going to cause a lot of issues IMO. I guess most sites out there (that use Gutenberg so far) are not prepared, and users are eventually going to the find the Template Editor and play with it, and it's basically going to mess up their layouts and thus sites. This is again a premature feature, shipped for the sake of getting it out there. I'm definitely going to have to deactivate it on all existing client sites, until maybe some day they will be updated to properly work with it.

I'd love to see more Gutenberg features shipped as opt-in, so devs can use those features in their new projects when needed, but are not forced on handling all new use cases on all of their existing sites.

I am definitely pro-Gutenberg and fully using it on all new projects, but it's just unrealistic to keep updating all existing sites with all the new features that come out.