In football the relevant metric is the differential physical ability at high altitude between players accustomed and unaccustomed to the altitude. Even if a player seems and feels fine, that would not rule out the altitude imperceptibly affecting his strength or stamina which would increase an apparent ability differential between him and his opponent. Thus, anecdotes from personal experience about people biking or walking or whatever at high altitudes are not relevant.Now, football is absolutely much more intense. But those guys are also younger and way more in shape than I am. Is there an effect? Absolutely! But you don't need anything close to 6 weeks to get significant benefits from being there ahead of time, either.
Furthermore, even if the effect normally didn't happen but only did sometimes to some players, that would still affect the play.
That's why in an other thread I requested statistical evidence for the effect on away teams of playing in Denver, but I did not get any specific information. (I would compare away team 4th quarter performance at Denver with their usual away performance, and correlate that with Denver away/home 4th quarter performance, compared to 1st quarter performance, as a start).