Every veteran so far has had an ample amount of work done to them. New animations, new special effects (slashing effects, magic effects, etc), and variable rebalance. Even if they didn't get new animations, every veteran has additional special effects for most, if not all, of their moves, so every veteran required a bit of work.
It's very possible that if anyone's cut, it's because they didn't have the time/resources allocated at the beginning to 'rework' every single veteran. Then, at the end of development, they had free time, and separated some alternate costumes into new characters to fill some extra character slots.
I wouldn't be surprised if, in initial development, Dr mario, and pittoo were just alt costumes/voices for mario/pit, like lucina may have been. They were later split off, had some variable tweaks, left the animations all intact, and gave us 3 'new' characters. Also explains why they're in the 'clone corner', because the rest of the roster was already designed/organized.