What's interesting, kinda, is:

1) Decoupling back from front also means the back because an agnostic service / provider so to speak. That is, you can, with some work (obviously), replace your backend and keep the same front. WP (or Drupal, or Joomla, etc. for that matter) become less usique, to some extent. Is this is a case of "be careful what you wish you?" :)

2) The white paper sez:

"Familiar backend for authors and publishers

One of the reasons WordPress has been so successful is that
it provides an easy-to-understand interface for non-technical
users"

Well...Um...Didn't Automattic just do their own remix of the WP backend? And isn't one of the selling points of the REST API that front and back end are more or less things of the past. We're decoupled on both ends. That is, a UI is a UI is a UI.

The point being, the WP admin's "shared experience" (read: familiarity) is, or should as promised, fade into something less and less shared (read: common, same, etc.) Yes, there are many benefits to this. No doubt.

On the other hand, gone are the days of a biz looking for, say, a marketer with "WordPress experience" only to find out that their new hire's previous experience (read: UI / UX in WP) have little to do with the new job. There is a real cost to onboarding and training a new employee. To date, WP had ROI appeal. It feels like that will erode as well, eh?

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"Yeah, I do WordPress," is becoming a more and more ambigious statement, is it not? Confusion, in the context of sales (read: buyer's decision making) and/or hiring, is never a good thing. Agreed?

From a technology POV the WP REST API is cool as f*ck, obiously :) What's not to love if you dream in IDEs and commits :) But The Market might not be so excited about yet another shiny new object going by the name WordPress. Perhaps WP is starting to mimic the mid 90's versions of Windows? That, ATM, didn't work out so well :(