With the exeption of touch of death combos (I'm looking at you Dante and Kat), long combos are generally balanced in this game.
Why you ask?
It has to do with the existence of combo versus zone characters. If played right a zoner can keep away a combo character for a long time, and shut down combos they start on others. Therefore, a zoner can rack up over 100 AP before the combo characters get close if they are good, and shut down their AP generation for a long time.
This means that when a combo character does get close, they should have the opportunity to lock you out of AP generation for about as long. Things get a little grey with the hybrids though.
Also, I main Jak and Radec, so I am infact a zoner.
I agree with your reasoning.
If you take two players of equal skill, one being a zoner, and another being a combo character, the zoner will keep the combo character out, but when they do land a combo, they'll pretty much equalize the AP gain.
Also, FP, G. Cole, Raiden, Dante and Kat have ToD's.
I'm pretty sure this wasn't intended, but if you combo it right, you can actually use Kat's gravity pull twice on the same oppoent and it won't eject them. I'm not sure how to do it, but I've died to so many black belt Kat's this way. The first time I got pulled in twice by the pull, I was like, **bleep**.