I'm a theme designer/developer but I work directly with end-users developing custom themes. I feel like I'm still missing something here. I've read posts linked from this one and other resources and still don't understand why .html files in the theme folder are desired for Gutenberg other than a strange notion to all of a sudden upheave the WP ecosytem for the sake of making theme development more accessible. If we're going to do such an upheaval, we should go towards a modern app development workflow that most professional developers would benefit from (and make WP less of a cul-de-sac of a career pathway for aspiring developers). It's funny that the working development branch is itself more in line with a modern workflow for the sake of the project devs.

Any way, why not continue with PHP files that are mostly HTML with PHP for dynamic content? If the proposal is for developers to work on the logic side of things in the WP admin, that sounds terrible–especially if there is no backwards compatibility for the current theming system.

As an aside, for my work, the block system is nearly pointless altogether (especially with ACF as an option). I never want to go through the trouble of designing a site just to empower my client to severely diminish it (which, anytime they go about rearranging the layout, is nearly 100% likely). Empowering the client to change content, not change design is our motto. Honestly, any time I setup a third party theme for a client on a budget, I always stick with the thought-out demo designs from the theme developer. Making a truly functional and beautiful design takes a lot of time and effort. Even then, theme builders end up making it much easier for the end user to accidentally mess up a design, even when they're just trying to change content.

It's also common for me to have clients that come to me after trying to "design" their site via some theme builder. They are usually frustrated and the results are lackluster, to put it mildly.

I get helping the SEO/Graphic Designer-type of professional be more empowered to create themes on the open source side of WP. But apart from that, the entire Gutenberg project seems like a way for Automattic to compete with Squarespace and geeky hobbyists to theorize about the democratization of the internet. Both impulses are understandable, but how long can one codebase support these end-goals and the goals of professional developers?