i would actually say the Sarge drew more heat than just about and heel i can think of for a short period of time. He was a turn coat siding with the enemy and burning the flag, i hated him as a kid and im not even American! He was an obvious transitional champ but he had the perfect gimmick to carry a brilliant angle, when America needed a hero the most Hogan came and took the title back. This in turn lead to Sarge turning face again, a great move which generated alot of cheers from the crowds.
Rude should have beaten Warrior no doubt. The cage match provided the perfect platform - the Heenan family could have seen to it but unfortunately this is something we will never be able to witness.
I don't think Slaughter burned the flag. On the Hulkamana anthology, he talks about how they wanted him to do so with the goal of really pushing his heel persona. Sarge said no; but, the brass kept persisting. So, Slaughter says he came up with a compromise.
Instead of burning the flag, he put Hogan's T-Shirt on a pole and burned it like a flag. Slaughter claims, back then, burning Hogan's shirt like that was as bad as (if not, worse than) burning Old Glory itself. And, it worked like a charm.
But, again, it was too short term. Once the Iraq war ended, so did Slaughter. In one year, he went from main-eventing WrestleMania 7 to being in a useless go-grab-some-popcorn-time 8-man tag match at WrestleMania 8.
You do have a point, though. Dibiase, Savage, and Earthquake's feuds with Hogan had pretty much run their course from '88 to '90. By '91, after Slaughter fizzled out, there were no major heels to face Hogan, besides Flair. And, since they were saving him for Hogan at WrestleMania 8 (originally), Undertaker got the top heel spot by default.