I used to believe that too, but reality has convinced me of the opposite. There isn't, and there has never been, a large (hundreds of millions of active users) online community that wasn't run by a for-profit entity. So the really difficult part, the one where the value ultimately comes from, does seem to lie in hosting and running such a community, and in attracting users. If that wasn't so immensely difficult and expensive, everyone would be doing it.
The demise of Reddit has been a long time coming, and now that it is actually happening, there is once again no plan for "the community" to move forward. The open source alternatives are all effectively worthless, with Lemmy apparently telling people not to sign up (yep, seriously!) because they can't handle the influx. No doubt Facebook & Co, or perhaps new commercial competitors, are going to pick up the slack as usual, because the real value is in the infrastructure and network effects that make platforms like Reddit possible in the first place.