Seeing as how the Expansion pack just came out, I'd say that there is still plenty of life in this game. And I really don't believe this is the last expansion for FFXI. I'm sure there will be at least another expansion before the end draws near for the game. But honestly, don't worry about the game dying or anything just as you begin to play it. There's still lots of people playing the game and plenty of people joining the game.

Anyway, there are always gonna be newbies just like you starting up and leveling in the newbie zones. But even if there aren't, there are 18 job classes in this game and higher level folks are always leveling new 1-10 jobs. There will be people for you to party and play with from level 10 all the way up to 75. Level 10 is around the level most people start partying and yeah, there are always people leveling in the dunes (the most popular 10-20 leveling zone).

Play the game. As long as you don't take things in the game too seriously (because FFXI does have it's fair share of drama and drama whores) then you'll have a good time.