One cool thing is that I had no idea that this happened until I just read it on their blog even though me, my friends and family use Matrix extensively.
But all my friends who are on Matrix host their own servers because it's quite easy, and my family uses my own server, only my brother uses a matrix.org account, but he doesn't write much.
This is decentralization working as it's supposed too, when enough participants are federated and not centralized.
On Lemmy the lemmy.world and the piefed.social instances are similar to matrix.org and I think all of them, including mastodon.social should close new registrations. I mean the flagship instances have their place in the beginning, but once they become so big they should be locked. The teams behind them can open another instance if they want to keep growing, but they should be run on a separate infrastructure to prevent bringing them down at the same time.