Actually, in a way, the current heroes are still in the process of fighting Thanos. Don’t forget that Dr. Strange saw only 1 in millions of scenarios in which Thanos is defeated. He willingly gave Thanos the time gem cause that’s what was needed to happen in order for the heroes to win.
The key was saving Tony cause he has an important role to play. Otherwise Strange wouldn’t of sacrificed his stone. (This despite him telling Stark earlier on that he wouldn’t sacrifice the stone/mission for him).
In essence, to triumph over Thanos, half the heroes had to ultimately pay the price (temporarily). Perhaps this causes a relieved and fulfilled Thanos to drop his guard....