I've never encountered this problem to the effect I needed a build step for HTML. I wonder if frontenders who work only with HTML/CSS find it helpful? Whereas they don't have the concept of breaking up code into templates or partials with a server-side language.

No disprespect to the author, what you've done is cool and your presentation is awesome. My biggest concerns are:

1. Onboarding & rampup time that's now added to my team just to write HTML.

2. Turning away potential hires who don't want to work with an over-engineered tech stack. Much opinion can be formed just by adding to your job requirements "we use a transpiled HTML processor"

3. Technical debt & tech lock-in. Will this technology be a detriment in 5 years? Do we get enough from it right now that it's worth future legacy costs?

It is, things like Slim are really helpful.

Over the years editors are doing a better job at showing you if you made a mistake, but it is still somehow cumbersome to write big HTML files.

Things like bootstrap really tend to complexify because you also stylize with divs

I don't think so. Certainly from time to time, but not on a daily basis; in my experience, it's writers/editors that need a decent HTML editing tool.