Mastadon instances are designed not to become large.
The platform is still in its infancy, but it's intended that people can migrate around the same way that you might move to a new neighborhood. And this is the feature that Twitter just fundamentally can't copy.
When you're talking about popularity, you have to distinguish between Mastadon as a platform/protocol and each individual instance.
I am (increasingly becoming) convinced that moderation does not scale - you can either have a single large community with no moderation, or you can have a small community with good moderation. This is what's clever about Mastadon. Moderation on Mastadon doesn't need to scale, because instances are fragmented.
Twitter will never be able to compete with that unless they split apart their service, which they will (probably) never do.
Those are pretty much guaranteed to go down the more people would use the service.
If you want to attract a lot of people, you will have to accept a lot of people, wether you consider their content crap or not.