With a very limited career of a few years and a staff of guys that are supposed to be coaches, why would it be a player's job at all to train the guy trying to take their job and end their career? I wouldn't do it at my own job if it would affect my long term job-health, yet we fans are single-minded enough to ask them to do exactly that.
How many of us would do it, help train a guy that will make you unemployed before you want to retire? The honest answer should be none.
Real world vs. football world isn't the same. I think you've mentioned that yourself a few times. Besides, Luck wouldn't be there to end Manning's career. Age, health, performance would. There are countless NFL players who took the young guys under their wings knowing they only had a few years left with the team or even in the league. I don't buy the no one would help train a guy argument. If Manning is the firey competitor that everyone claims he is, then he should have no problem with them bringing in a replacement, especially at this stage of his career.
Um, then why would Luck be there? He'd only be on the bench as long as Manning can hold him off.
Pro athlete: practice your job to keep getting better or spend time teaching the young guy who's there to ultimately take your place?
We aren't talking about a Steve Deberg or Mark Brunell, we're talking about a HOF caliber QB who has to firmly believe he's still the stud QB to lead a team to a title.
Also, let's realize why great athletes make crappy coaches: they do a lot instinctually and it's not easy for them to verbalize what and how they do it. Reggie White couldn't even teach that hump move of his no matter how much he tried.
As Jim Finks said "Owners own, coaches coach, players play."
It's fans wanting to believe that their sports heroes are such great guys that would do anything for their favorite team, like train their own replacement.