Yes, folks, if you don't like the idea of a corporation taking your ("software-as-a-service") code and using it to create and sell proprietary products, then maybe don't license your code under the MIT License (or any of the others which explicitly allow exactly that)!
I apologize for stating something that's common sense but apparently, as they say, "common sense ain't so common".
The MIT License is my preferred Open Source License but the AGPL is completely acceptable, too. In fact, if you (intend to) form a legal entity and sell your code as a SAAS product, the MIT License is quite likely NOT the license you should choose!
I don't blame these guys a bit for switching to the AGPL now but I can't stress enough that selecting the proper open-source license is an important decision for any "serious" project. Please make sure that you understand exactly what the terms of your chosen license really mean before you decide on it -- and if your stated goal is to "grow a sustainable open-source project", well, the MIT License (and similar) is almost certainly not the best choice.
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ETA: Also, props to Plausible for choosing the AGPL as the license going forward. As mentioned, going closed source would have been the "easy choice". Alternatively, you could have selected something like the "Commons Clause" or one of the other recent "open-source but not actually open-source" licenses. In my opinion, the AGPL was absolutely the best choice. I've never used your product -- or even heard of it before now -- and probably never will but thank you for your contributions to open-source nonetheless!