I forget where I saw the numbers, but there are still over 1 million individual accounts across all servers. Bear in mind most of those are likely mules, but even if each character only had one mule, 500,000 players still isn't small by any stretch (it isn't Guild Wars or World of Warcraft, but meh).
Last consensus put the population at <500,000.
As for how long an MMO can go? FFXI's peak beat EQ's peak, and EQ's still has a live development team.