Even if they hit the cap Blizzard did promise an end game hero class that people can start advancing in AND they still havn't given us the info on how all that will work. Not to mention they are still thinking about designing a life quest that each character can attain that will take him almost of of his/her "life" to complete.
Toss that in with the higher end raids, pvp, and events....there should be enough to do in the next year or so without worry of boredom.
I played a year in FFXI and I couldn't take another second of it. I had several jobs to 30 and my highest was 45 DRK. Lots of work, gil, and items that some lucky peeps got for free but damn did it feel good to quit.