Poor script and poor concept that 'changing past doesn't change the present'

Rating: 5/10 (4 May 2019)

All 6 stones took from various positions at time line and collected at 2019. Also Thanos came to 2019 from 2014. After the fight Cap returned all 6 stones to where the brought. ( It should be teseract and Loki's wand. Not the stones. ) But Thanos vanished at 2019 means no return of Thanos to 2014. It means there is no effect from Thanos after 2014. No first snap from Thanos. It means no endgame at all.
Okay then. If changing past doesn't change the present then why the hell Tony and Natasha can't bring back.