It's freemium, but the premium accounts are actually worth buying. You get more upload time, better stats, more customisable embedded players, etc. Lots of people buy them, I assume it's already quite profitable. The "prosumer" market for music production is expanding enormously at the moment with the american EDM boom.
If twitter shuts it down or ruins it, there will be blood. A lot of people depend on it and there would be instant demand for a replacement if the userbase turns its nose up at any changes.
OTOH anyone promoting their music on SC is almost certainly already doing so on twitter as well, so some simple, well executed integration could go a long way.