Literally who uses more than four or five of those saves themselves. Hey, here's a tip - if you have a beginner game and you want to start another beginner game, save over the previous game! Sheesh, I literally don't see why this is even slightly a big deal. You can rebattle more difficult versions of bosses in these portals, and if you don't want to do that you can literally just play through the game again. Didn't BbS not have this 99 save system anyways? I don't remember any hearts stopping that day.
BBS had infinite space, based on the size of your storage unit. 3D only had 2 or 3 spaces, CoM only had one until you beat it, then it gave you a second for Riku. I don't remember Coded or Days original save slots but I think they were only a few, and 1 and 2 had 99 options but it was based on memory card space as well.