Many people seem to call Jotaro learning to time stop in the end of stardust crusaders a asspull, but i don't think so. I think Star Platinum's real ability is self-preservation, adaptation and development potential. Star Platinum protects the user from scenarios hes unaware of, like when jotaro was about to shoot his own head.
Star finger was another example of Star Platinum adaptating, in the battle against dark blue moon, because dark blue moon was far away, it developed the Star Finger Ability, which allowed him to extend his fingers onward and hit the opponent.
Heck, you could even say his enhanced breath was another adaptation so it could beat justice.
Dio had time stopped and because jotaro couldn't protect himself, Star Platinum kicked into action and developed the ability to move in time stop, which slowly developed more as jotaro noticed he could move and react to dio's time stop to the point where jotaro himself is the one who stops time.
Notice how on later parts star platinum never uses star finger and mostly relies on time stopping and punching the opponents, i think thats because jotaro didn't care about developing the rest of his abilities, because stopping time simply resolves most of the situations.
this reminds me of how dio didn't care about his vampiric abilities because of time stopping.
Yeah, you could also say "Araki forgot" and im gonna say you aren't wrong about that. I just really wanted to state this, my opnion on star platinum's ability.