Hot take: closing registrations on this instance will have been the secret sauce needed to make federation really take off, and others should follow pattern. Lots of instances with limited registration numbers is a good thing for now.
@gar Yeah, that's what Evan did with identi.ca when he switched it from #StatusNet (née laconi.ca, now #GNUSocial ) to pump.io. The switch has been debated hotly, but I think it's clear that the closing registrations part had the intended effect on the pump.io world.
I would say it partially worked. Identica started with such a huge number of users that I doubt any other instance is even close. When the #bifurcation happened, many people stopped using their early pump.io accounts and went back to ica. But the imbalance can't get worse because no registrations allowed there.