I mean short and long term there’s pros and cons to that. however there’s a reason why that started to fall appart with e-mail. In short if it gets popular, than hosting servers with no throttling or post limits means spammers are going to go crazy, and rather than play the never ending unwinnable whack a mole game as bad actors create thousands of instances a day, hosts of any instances worth targetting will have to do a “instances are assumed malicious until proven benign”, (IE a whitelist method)

Any instance hitting one million is unlikely, on the mere grounds of trying to make one super instance is kind of the opposite of the goal of federation. The winning would be reaching a million members between all instances.