#1. Roman Reigns' feud with John Cena would have had an entirely different dynamic
In 2017, John Cena had a feud with Roman Reigns. While it was a match that most fans felt would be saved for WrestleMania, WWE felt that it was better to have a marquee match at No Mercy instead.
In the whole build to the match, there was no physical contact. But there was a verbal burial done by John Cena. If you recall the August 28th episode of RAW, John Cena humiliated Roman Reigns to the point of no return:
A female WWE star said her life is in danger. Details HERE
It was clear that Roman Reigns wasn't comfortable in his role, and John Cena called him out on it in front of millions of people. Had Roman Reigns turned heel after WrestleMania 33, he would have been much more confident, and his verbal back-and-forth with John Cena wouldn't have been so one-sided.
Roman Reigns was always going to win the match, but the entire dynamic of the rivalry would have been different and better had he been a heel.