Personally, I think that this is a good choice-- the editor system in that CMS is a very good use case for complex JS technologies.
But at the same time, there are a whole lot of of folks who have been doing a lot of work with PHP who didn't want to learn that toolchain.
In that context, it's a little complex: we have a whole ecosystem of businesses and developers and software tooling that is being pushed over by some core changes. I'd bet that there are going to be a lot of frustrated low-end PHP devs dragged into learning modern-ish JS tooling.