Interesting. I'd expect something entirely opposite - with one platform, pay-for-entrance, shills / marketers can just write the entry costs off as a small marketing expense. OTOH I can easily imagine a lot of smart people with interesting things to say shying away from spending money on such a platform.
I assume that $5 a month is only a problem for occasional users -- but I doubt that those add a lot of value to the ecosystem (aside from higher numbers of total users signed up and other vanity metrics)