The Reigns hate was somewhat due to poor scripting, but a real talent could fight through such failures. He has had three Mania main events and hasn't made a real dent. If anything his fight against Brock is actively keeping fans who gladly spend thousands on travel from going because he sucks so much at basically the fundamental core of wrestling (and workrate isn't it): playing a character. Those cheers he got were cheer futures, based upon the belief he'd be the next guy and would elevate wrestling to a cool and interesting level.
On the what chant as much as it sucks it is based around wrestlers following a script that made no sense. Instead of doubling down and working harder, like during the MNW, the company just pushes forward -- future be damned.
On faces being booed and heels cheered in the 90's this was a much more common element. What fans are doing is reacting to the quality of the show. By booing Maivia they are saying stop this, and thusly the company gave us the Rock. No more boos. When making him a main eventer they needed him to be heel, even though very popular, they still got him boos. All they had to do was have him bash a lovable babyface and attack the fans. Using it is what was done in the past. The entire Hulk character (invented in Gagne's AWA) was seeded by the random audience cheering Hogan's first appearance.