People forgetting this always baffles me. I get not remembering the details of the Colorado game. I had forgotten them myself for years until I looked it back up some time ago. But nearly everyone forgetting Major's multiple serious injuries is bizarre - especially considering people were forgetting about it while he was still recovering.
Remember in 1999 Chris was the backup all year until he started the A&M game (also the Bonfire Game) after Major was sick with food poisoning thanks to Dinesh Patel. The game was a defensive struggle and Chris didn't play great, but we were up 10 at halftime. At that point Major must have said he was feeling well enough to play so we swapped QBs, and then put 0 points on the board (only one drive even gained double-digit yards, and it ended in an Applewhite fumble) and lost by 4. IMO, although it's impossible to know for sure, Major just still shouldn't have been playing. Not Major's fault obviously, but ironic that arguably the first mistake Mack Brown made in the Major/Chris situation was to pull Chris and put Major in.
Then at the end of the 1999 season came the Cotton Bowl, where Major blew out his knee really badly in the first of many examples of Greg Davis being clueless of how to beat the blitz. In summer practices it was pretty obvious that Major was not 100%, so Chris got the start in the first game. He promptly threw a pick 6 and we were down 10-0 to ULaLa after the first quarter so Major came in and we won 52-10. I think at this point we all thought Major was fine, but he wasn't, Lafayette just sucked. Mack had to choose between a healthy but still inexperienced Chris, and a less-than-full-strength Major, and both struggled at times. We lost to Stanford, who finished the year 4-7, even though we had beaten their 1999 team that made the Rose Bowl by 52, and they both played poorly. We'd switch back and forth whenever one guy looked bad and our fanbase was tired of it and Mack was getting blasted whatever he did.
Then finally Major started looking like his old self again and look over the full-time starting role. He looked great and our offense was clicking. It lasted 4 games and then he blew out his knee again in the 3rd quarter against Tech and was done for the year. Simms played well for most of the rest of the year, including a 43-17 annihilation of A&M, until throwing 4 picks in the Holiday Bowl against Oregon. In spite of that, he brought us down the field in the final minute with a chance to win, and threw 3 consecutive accurate passes into the endzone which were all 3 dropped by our receivers.
Come to the start of the 2001 season, and once again Major still doesn't healthy in the summer. So once again Mack Brown has to choose between healthy Chris and hobbled Major. Given how the previous year's QB shuffling hadn't worked well on the field nor off the field, he resolves that this time he will pick one guy and stick with him. Presumably because this time it's no longer a "healthy but inexperienced" Chris, or maybe Major just simply wasn't ready yet at the start of the year, he goes with Simms as the starter and sticks with him.
