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cause the stones got destroyed

Only one reality existed where Thanos was defeated.

There were 14,000,605 possibility Dr. Strange saw but there is only one where mad titan can be defeated which is the sacrifice of Tony Stark.

The thing about time is even though it’s malleable it’s probabilities are inevitable. Saving Tony would lead to an alternate time or alternate probability of the reality so results would’ve been different (and there’ll be consequences of the time travel done by Avengers to bring stones too even though movie showed that returning stones to the time they had taken from would do no harm but it’ll definitely lead to an alternate timeline without letting people of this timeline know). Surely he’d have searched for events whose causality wouldn’t have resulted in demise of tony but sadly he didn’t found any such events so he signalled him that this was the only way.

There are more than 14 million possibilities and we win only in one and for that one to happen doctor starnge didnt saved iron man as saving him would lead to change in future