I actually think, if Aerith had lived, the CloTi ending would be even more definitive, regardless of Aerith's chemistry with Cloud.
The Lifestream sequence, in any other story, would have ended any other possible "relationship" for Cloud and any other girl. That was the pinnacle of Cloud's story arc and only Tifa could have done it. In a story where Aerith lived, the Lifestream Sequence would have been a scene much like the Ragnarok sequence in FF8. There wouldn't really be any room left for a third-party female once Cloud and Tifa emerged (together) from Cloud's now whole mind, no matter how much chemistry Aerith might have had with Cloud before this. She, like other love rivals before and after her, would have had to respectfully bow out at that point.
Had Aerith had lived, she would have had to fight Sephiroth alone once Cloud went nutty because she'd be the only one who could stop the impending doom of Meteor as a Cetra--only Tifa would have been able to stop fighting to nurse him. Even if Aerith wanted to give up her duty, she wouldn't have been able to because of the responsibilities she must take as the last remaining Cetra--things only she can do.
The only way Clerith actually works is if the story remains as it is. Because only a dead woman can manage to "win" over a living woman who has put a man's mind back together--or can be imagined to be "winning" over that living woman.