I've been playing since the beta and have over 2000 hours in SoT. The community is plenty healthy and there are always plenty of people to play with, with plenty of SoT groups, clans, etc. There are always 100-300 Looking For Games (LFG) posts for SoT. The only part of the game that is a little slow is the matchmaking in Arena mode for galleons, as there can be up to five galleons with four players in each and not everyone comes in with a full crew, so the matchmaking process can naturally be a little slow. Arena mode for sloops is much quicker.
Not every multiplayer game needs to do WoW, COD or Counterstrike numbers to be successful, have a healthy community and be profitable. Right now, Rare is working on new content for SoT, expanding the map, adding new islands to explore and new events.