Very good points, Justin! It's like we took the old page builder lock-in and just moved it over to the native editor. I guess that's better, but still something we need to think about.

As I've built more client themes with Gutenberg in mind, I've tried to find some ways to work around it. For instance, I went from creating custom ACF Blocks and sticking them into the theme to creating a custom plugin that allows them to survive, no matter what theme is used. Any custom block CSS is also included in the plugin.

Developers are going to have to consider these things. But it will be interesting to see how Full Site Editing impacts the theme market. Maybe you're right and people won't see the need to change themes as often.