As jwodder stated, the official original answer was just that: they never though about it.
That said, after the season 7 finale there is a new possible explanation that actually would give a meaning to the original spell wording. Mind you, this is still not confirmed, and would at best be a retroactive explanation they added long after the original episode was written.
In the S7 finale we finally discover the reason Star Swirl went missing. He sacrificed alongside the other pillar in order to bind the Shadow Pony.
Now, consider the words of the spell:
"From one to another, another to one. A mark of one's destiny singled out alone, fulfilled."
While S7 Star Swirl seems pretty stubborn, he also seems to value friendship (if you exclude random fights over misunderstandings.. ). A possible interpretation of the unfinished spell could safely be an attempt to craft a spell that would sacrifice only him, leaving the other pillars to live their life.
destiny singled out alone, fulfilled.
That is, his destiny would be to be alone forever in limbo, having fulfilled his task to protect the realm.
As already stated, this is obviously just a possible interpretation and was never intended back when season 3 was made. Still is possible that this is an example of the show trying to fix its loose ends at a latter time, so it is probably worth noticing.
Also worth noticing is that the above explanation would give a second meaning to the fixed version Twilight made.
"From all of us together, together we are friends. With the marks of our destinies made one, there is magic without end."
This weird enchantment is oddly fitting the season 7 finale.
"All of us together", the new elements are together with the old ones
", together we are friends", the pillar become more than a team and now real friends.
"With the marks of our destinies made one,", the destiny of the pillar now is intertwined with the destiny of the new elements bearers.
" there is magic without end.", their original banishment has now ended and they will now live once again in friendship (== magic).
Basically, if you accept this interpretation, it is possible to argue that Twily fixing the spell set the cogs of fate in motion so that the pillar would be saved someday.