From a quick search, it looks like they raised $21 million in funding half a decade ago (after an earlier seed round), so this seems less like a reflection on the nature of open source in general and more just a reflection of this one project. It was set up in a way where development was happening because people were being paid to do it, and this time next year, they won't be. Maybe the community would have come in to try to keep it going if there wasn't any funding, or maybe it would have just stopped seeing any real development, but we can't really say for sure either way.

the $21M was presumably not just to develop deno but to build a business around it, which is a much harder problem. this feels like more of a bystander effect problem; if as few as three large companies that benefited from deno were willing to form an "openedeno" foundation and assign one full time employee each to it i'm betting that would be enough to at least sustain the project in a usable state, but of course everyone is hoping someone else will do it.