For smaller group, people updating attendance is really useful. I've cancelled meetings when not enough people could attend, or change venue if more than I initially anticipated acknowledged. This is not easy to do on a simple github.io style static site.
oh, of course, but it doesn't have to happen on meetup.com, you can use any other service, from the many suggested in this discussion.
of course having two sites is more complex than having it all-in-one. that was the whole benefit of meetup.com, but that benefit is now going away.
one reaction is to reduce the dependency on meetup.com for groups. we need meetup.com to help get the word out, but we can use other tools for the actual group management.
the only thing that virality helps with is event anouncements. finding all events in the same place, that is what makes meetup.com useful. the actual registration, and what not can be done anywhere else without causing to much friction.