1. It is not debatable that games have been "fixed" in the past in all major sports.
2. The term "fixed" can mean very different things. For instance, a FG kicker could intentionally miss two kicks. Does that mean the over still won't cash? no. Does it mean his team might not still cover? no. But it greatly increases the odds of those events. In other words, the game may not be fixed in some instances to a 100% result, it is just influenced to make the desired result far more likely.
3. There are ways to limit your exposure to "fixed" games. For instance, if you are a successful originator and a line keeps moving away from your number, and by kickoff you have the variance needed to make a wager, you may well be betting into a fixed game. You can either limit your bet size or avoid these types of plays. Because one thing is almost certain, the line move will coincide with the "fix". You often hear someone say "Vegas called" but rarely do you see Vegas on the correct side of the suspicious game. How could they be, when the fixers are pounding the side that it is fixed for?
One thing people fail to realize is that in Professional sports, it doesn't take much to heavily influence an outcome. The NFL lines are probably only less tight than major soccer lines but they are super tight, however you want to look at it. A 1 pt move in the NFL is very significant. How hard is it to influence a side or a total by just a couple points? And that is all that is needed to make good money.
I would say the most obvious football game fix that I ever saw was this gem ( consolation prize to San Diego/Pittsburgh ):
https://www.espn.com/college-football/game/_/gameId/303020041
If you want a good laugh, you could check out the line movement of both the full game and halftime spreads. Hilarity.