Well, it's just the free speech issue all over. Either you censor speech or you risk people saying things that you really don't like. This is exactly the same situation: Either you go FOSS, with all the benefits it entails, and risk getting forked by a major player or you go proprietary and risk not getting adopted at all by major players and open source enthusiasts.

Don't get me wrong, I feel with the Elastic team and I can understand why they're unhappy (even if I don't agree with their pettiness). I also don't particularly like the move Amazon did here. But that's why we want free software: If the current owner of the software does something you don't like, you can still use the software. That's the whole point.

And you're still free to pay Elastic to manage a cluster on AWS for you; Amazon might have forked the project, but it's still the customer that needs to switch to OpenSearch.

Okay, but what about something like BSL, with the older versions automatically getting fully open-sourced?

Not technically open-source, but much closer to open-source than what we would generally think of as "proprietary" software

The end result may be that there's a little bit more proprietary software; so be it, people could already make proprietary software. We'll still have plenty of Open Source software that's actually Open Source. That's better than losing Open Source altogether.