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A lot do.
It's just time.
If they survive the first few years, they will grow organically constantly.
The community of communities is still a sound idea, the compounded organic growth of each community does create significant growth.
The flaw for ventures walking this path, is that the time it takes for the compounding effect to happen for new sites is too long for the life of the venture. Especially given that large communities tend not to migrate to different tech... preferring to stay with whatever is working for them now, as the effort of moving is seen as worse than stagnation.
...and very few community / forums achieve scale...right?
Even if you have a community of communities