Abyssea would've been a complete fail without being able to level multiple jobs fast. If you think about all the things you could obtain for so many different jobs, if it was just an added event like dyna or sky, not enough traffic would go through there. AF3 would take forever and a decade to obtain. Not to mention it would've taken a decade to discover all that had to be done and found there.

Say you saw Cor gear in abyssea and decided to level Cor. For some it can take 6 months or more to obtain the 65 cap you're proposing, then they can hope their ls is still interested in doing abyssea by the time they reached that. After you achieved getting that, let's assume you wish to do another job. Another half year to get 65 and more than likely abyssea isn't nearly as popular since we've had to have had a new update. Repeat depending on how many jobs you wish to level.