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I want to offer a more community- and not topic-orientated answer to this question. Basically, you could rephrase the question to  discuss any topic. Many topics have vibrant sub-topics. Mathematics is not just mathematics, besides algebra, calculus, there also are (applied) fields like statistics and game theory which itself is tightly knitted with economics.

Oftentimes, there are no clear lines between different sub-topics or even topics at a superordinate level. Think about medicine and psychology: Even though there are many questions that can clearly be assigned to either field, every question about, for example, psychosomatic disorders could be (partly) answered by a physician or a psychologist.

Many smart people spend a lot of time day in day out to categorise stuff, objects and data. We're doing the same right now. Some classifications are easy, some are hard, some are so questionable that they lead to misunderstandings or irritated questions. (And usually, as soon as you categorise anything, somebody already has a better idea to categorise it or at least claims to have a better idea.)

Maybe we should start out with a single site which is home to many or even all sub-topics. Let's see how it goes. This has some decent advantages:

 * It eliminates possible confusion from new users as for where they should post their question.
 * It keeps the workload low. Moderating one site is easier than two or even more.
 * A single site allows for an easier interaction with users, meaning that feedback is not scattered in five meta-sites but focused in one site.
 * In the end, the community that's there can decide what should be happening next. Many machine-learning questions? This should be an extra site.

Right now, we don't know how much participation is to be expected if we start a single site, two sites or any number of sites. Like on SE, it's always a gamble to create a new community - sometimes it works out fine, sometimes the start is promising only for the site to be slowly drying out from low user engagement. So instead of betting on multiple horses at once, it may be more sensible to bet on one for the time being.