Maybe I'm missing something, but I don't really see the sense in the "pull out the rug, it's not OSS" anymore. Sure, I agree that The Open Source Definition doesn't encompass something like the Elastic License, but :shrug:
Who exactly is having the rug pulled out from under them? AWS? As they very clearly state in the article: "When AWS decides to offer a service based on an open source project, we ensure that we are equipped and prepared to maintain it ourselves if necessary" (presumably because they understand that the license could change, or the maintainers could just stop maintaining).
It seems like it made good business sense for Elastic to switch licenses, and it's not really clear to me who would (for business reasons, not philosophy) switch to solr/other just because of the license switch.
It also makes perfect sense that AWS would release an apache-licensed fork - they have serious monopoly power - they con offer this service cheaper inside AWS, and the integrations can be smoother/better.
So really why be mad at either party? They both made good business choices it seems to me, maybe elastic will end up loosing out, but if that happens, I can't imagine that staying on the apache license would have saved their bacon.