Wasn't too long ago SM64 got a new 120 star WR from cheese, several months ago I think? SM64 is practically its own genre from ROMhacking now so it'll never die. People don't just do base game, they do various categories of games like Star Road, The Green Stars, I don't think people actually run Last Impact, and just beating ROMhacks often requires similar skills as speedrunning the games because people get so creative with designs.
Most popular games I'm into I've at least learned a little bit about through speedruns. If I like a game I tend to look for cool stuff about it and speedruns are a good source of that. SM64, OOT/MM, SMS come to mind in particular though since they're extremely popular and well-developed speedgames close to my heart. I haven't watched a whole lot of SMS runs though so I'm sure there's a lot I could still learn about it.
'Official' Plusle of the SSBU board. Also, creator of that thing down in the line under this. http://www.gamefaqs.com/boards/997828-paper-mario/57781633