EVE Online was released six months before World of Warcraft and has only within the last year or two cracked 350k concurrent subscriptions. Six years ago when I first started playing it you'd be lucky to see 25k players on in any given night, and the average player population in the game was closer to 10-15k. Now the numbers are closer to 40-45k and 25-30k respectively. EVE Online is a $15/month subscription-based game. If anything EVE Online proves that so long as you can carve out a niche of dedicated, loyal players, and don't squander the money, you can continue to support and develop a game even with "low" subscription counts.
Rift has a decent shot at staying alive for years to come so long as Trion doesn't screw up by the numbers.