that's what the Ancient One told Banner. that's why if someone where to obtain all 6 stones they could alter reality. that's why they were able to keep everyone from the past 5 years and still bring back everyone who had been dusted. they weren't different timelines but just different points on the same timeline. the only way that Cap could show up at the end as an old man in the same place where he left is if there was only one single timeline. if Cap staying in 1945 created a new separate timeline they would have never seen him again.