> Maintaining a fork of WordPress with its own independent development priorities would involve a lot of work.

What if they had no interest in developing new features, and just concentrated on security and bug fixes for the current version?

I'd expect that most of the people who use a site like WP Engine to host their WP sites want to put out their content in a nice format and once they are happy with the way it looks and the tools they have for editing and administering it they would be would be fine with it staying on the same WP feature set for many years.