On the contrary, this group has been bombarded with submissions pretty regularly - it was I who was inactive.
When the singular staff member poofs, the group grinds to a halt. Regular members don't get to see the hundreds of artworks that expire and never make it to the gallery. If I can find a couple moderators, we'd never have that problem.
You are right, the fandom is not explosive anymore, but it's also still immensely active when it comes to fandom content. One look at Derpibooru or Fimfiction will tell you that.