The problem with these sorts of discussions is both sides always want to have it both ways. When the complaint is they aren't doing something and that's against the spirit of the thing, the response is "show me the contract that says I have to do it". But when it comes to talking about how their opponent is being unfair, the words of the contract don't matter anymore. That's just not how things work, either the contract is all controlling, or the relative behaviors and non-contractual expectations matter.
Matt might well be throwing a temper tantrum, and almost certainly causing brand damage, and the result of the conflict might be actual harm to end users. BUT WP Engine doesn't get to hide behind the limits of their obligations under the open source licenses and then shift the blame they should rightfully be taking onto Matt because of his temper tantrum. Real users are not getting security updates or features they expected because the company / vendor they are buying their product from did not do due diligence to secure their supply chain. Matt could decide tomorrow to stop releasing new WP versions, or change the license (modulo CLA stuff). It's not like sudden, fundamental changes to the upstream licensing / sourcing hasn't been a constant source of headlines and conflicts for the last few years now. CentOS, Redis, HashiCorp, Akka, CockroachDB, and many more projects have fundamentally "altered the deal" and downstream customers relying on them have been caught in the crossfire. Heck, even the GPL2 vs GPL3 debate is an example of this. Are all the projects that switched to GPL3 for anti-TiVoization clauses guilty of throwing temper tantrums? Plenty of real world users were harmed by moves to GPL3, for example, Bash on macOS is stuck at 3.2 and users were forced to migrate to zsh over this move.
> There's no way to operate these features without WordPress infrastructure
Is WordPress not open source? What stops WPEngine from doing it themselves, they have the source. If its too hard, well that might explain then why their upstream vendor wants some compensation for the work. We (rightfully) criticize commercial companies for not putting resources into the huge numbers of open source projects and labor that underpins their very existence. Well this is another example of that. If an upstream source is so critical to your business that its loss would cripple you or your customers... maybe consider spending some money on securing and retaining access to that source.