They will not create their own instance, they will come to ours. And we don’t have neither computing power to support all of them, nor enough skilled moderators to respond to all that, not enough of a routine and etiquette to quickly understand whom to kill and who is there legit.
I can see it all evolving over time, but I am really not optimistic about the scenario of Lemmy trying to accommodate to even a 100k people overnight. And reddit is much bigger than that.