Interesting. We could move to a model whereby the .html theme files are, in a sense, a custom post type similar to the difference between Pages and Posts in a standard installation.

With standardised .css calls and block names, a change in theme would only require a change in stylesheet. I suspect a really good theme designer would then have to explain how s/he designed their sample theme, which blocks they used where, so that users know how to get the same experience themselves.

So... yeah. Interesting.