Google Groups has survived 22 years. How many self-hosted Discourse forums can say the same? If you're working on a new, small side-project like a new formal evaluator, are you going to want to spend your limited administrative bandwidth onboarding new contributors, responding to feature suggestions, talking to users, etc, or are you going to want to spend it trying to wrangle self-hosted forum administration and fig…
Look at Slack's organic growth - it was great, people just started it and basically forced adoption on companies. Not so much today, because things evolved again. Discord is doing the same - people just start one, doesn't have to be official, then ultimately, it becomes official.
But you know, if your FOSS community can only survive with one specific tool, is it really a community?