In reply to Nick.

Most of us develop sites in a localhost environment. When we use FSE to create or edit template pages/parts, those designs do not get written into files, but get stored in the database...

I would not recommend that theme shops, agencies, or freelancers build themes in this way, at least not at this point. I certainly wouldn't. It seems like a recipe for a lot of headaches.

Last thought, are boxed themes even possible with FSE?

Definitely. I just boxed my theme in about 30 seconds:

Agreed on a lot of the weak areas. Margin/padding controls are limited to just some blocks right now. There's a ticket for a global spacing value, but I doubt that makes into 5.8. All of those foundational issues are going to play out in patterns.

I have more thoughts on the foundational issues creating theme lock-in. At this point, I don't necessarily think it's a bad thing -- or, at least, I've given up on my hard stance against it. That ship sailed with the launch of WordPress 5.0. I'm not sure if there's any turning back. But, it's late. I'll consolidate all of those thoughts into a more coherent and organized post.