It's useful for local events, but very expensive for organisers ($15 a month for a basic events page). People pay purely because others use it, an open alternative would be 👍
Meetup's "network effect" might be overcome because it isn't a global network (e.g. someone in London is only going to care about meetups in London).
Also, an open alternative would be perfect fit for #federation, each community having its own instance.