Time will tell if TOR will be around for the long term. With everyone leveling so fast, it is anyone's guess.
My main concern is the speed at which you gain levels. While this is great to get power and abilities quickly, the game has a max level of 50, and I am already 25 after a few days of play (and only playing a couple of hours each day). You get a lot of experience for everything you do: quests, flashpoints, pvp, space combat, etc. which is great but makes you pass by story and zones faster than you probably should. Heck, this is only one day after the official launch and I see people leveled to 40 and more running by. If everyone is able to get to 50 with even the most casual play in a few weeks or so, what do you do next to keep interest? There is always leveling another character, or running instances with a guild of friends at 50, or pvp, but I am not convinced that is going to be interesting enough for a lot of people past 6 or so months. To make the comparison to the current MMO king (like it or not), it took me months to level to 60 in vanilla WoW. I was a pretty casual player and tried a few different characters, but I don't see that it could ever take that long in TOR.
I could be wrong because there are a lot of interesting and fun things to do in TOR, but most are side tangents like crafting and the group instances. Bioware has done a lot right in the game, so I am willing to give them the benefit that they will be able to come up with something sustainable once everyone has hit L50 (in like a couple of weeks from launch). There are also of course 2 L50 operations (raids) that will keep raiding guilds and players interested for a bit more. For casual players there isn't a dungeon/raid finder so you will be forced to use trade chat to try and find a PUG dungeon or raid once hitting 50 (and before). There has been talk of them making a better space game (the current space game is OK, not great) and putting in more battlegrounds and such (I like 2 out 3 of them a lot, but Huttball is pretty awful).
Time will tell if Bioware is able to keep subscribers and make the game viable over the coming years. The biggest factor will be if they can crush the lingering bugs quickly and get new exciting content out the door before everyone gets bored of the same old thing at 50.