The problem with WordPress (and it looks like this solution largely just replicated the problem) is that it's way too cumbersome and bloated.
It really is unlike any modern UI for really any SaaS or software in general.
It's filled with meaningless admin notices, the sidebar is 5 miles long and about 98% of what the user sees is meaningless to them.
Creating a very lightweight, minimal UI for the client to edit exactly what they need or like you said, just static files really is the best solution in most cases. The "page builders" always just turn into a nightmare the clients end up handing over for a dev to "fix" anyways.
Not sure why so many people feel the need to continue on the decades of bloat and cruft WordPress has accumulated, even if it's "modernized."