Take ROG/NIN as an example and what comes after Spinning Edge. Depending on which debufs are already in place (and on whether any skills have been lost to level sync) I may want any of the following combo chains:

Spinning Edge > Shadow Fang
Spinning Edge > Gust Slash > Dancing Edge
Spinning Edge > Gust Slash > Aeolian Edge
Spinning Edge > Gust Slash

So, if it were possible to make combo slots, I'd still need 4 slots worth of Spinning Edge combos, and likely a 5th slot for a command to reset them all back to the beginning, for cases when I'm doing something else long enough for the combo effects to wear off, or an attack failed so its combo didn't trigger. Five combo slots for the various ways to combine five actions wouldn't really be saving any hotbar space.

I'd also want to be able to wait, for instance, until after Gust Slash to decide whether I'm going for the Dancing Edge or Aeolian Edge combo, especially if I'm switching between targets. That means all four of the combo slots would need to update according to what I'd done in the other three. It ends up a lot messier than just having those 5 slots for each of the individual skills involved, as we have now, where I can use and combine them however I wish.

(Similarly, I could replace my slots for the Ten, Jin, Chi, and Ninjutsu skills with slots for Fuma Shuriken, Katon, Raiton, Hyoton, Huton, Doton, and Suiton mudras, but replacing 4 skills with 7 doesn't exactly save space either, not to mention the fact that it would completely eliminate the whole mudra forming mechanic from the game.)