Yup, they have the userbase already. Also, they disrupted the incumbents by being an order of magnitude cheaper (or cheaper than that, based on how many songs you're selling) than their competitors, which helped their userbase grow.

It's hard to imagine a new competitor being able to compete with DistroKid on price at all, considering how cheap they are. An order of magnitude cheaper is probably out of the question. And these numbers (for the larger clients) are peanuts on DistroKid anyways, so even if you could compete on price it might not matter for much of the market.