Id love to see what supporting evidence you have make this claim accurate. It seems that you are operating off of the assumption that these players would not replace all of those football hours, which would be in the 10s of thousands, with any other proffession training, education, or in field experience.
These are no different that any of us, and we (most of us) have careers in a wide variety of fields, so why wouldn't they? They had different paths to their jobs, but it doesnt mean that they couldnt have ended up in a completely diffferent professional space if they made one decision differently. Look at Laurent Duvernay-Tardif... he is a doctor first, and pro football player second... that alone destroys your narrative of "nothing else they can do"
Also, if you think NFL players make "that kind of money," then you should read up on how the salaries really work out for these players. Kids come into the league broke, and leave the league broke more often than not. Over 60% of NFL players dont even get a 2nd contract, and over 15% of the entire league makes less than 200k over their entire NFL career (thanks to the new practice squad system).
The average working american makes more money than the average NFL player does over their career. There is data to support that. Here is some math to help out:
The average NFL players career lasts 3.3 years with an average salary of 860k, and since their tax bracket is of the highest, they are taxed 37% of their total income. These contracts do NOT include any long term benefits, retirement benefits, or disability and life insurance. Those all cost separate.
So with that, lets just leave out cost benefits, and go with salary post tax... That means the average NFL player takes home 541k per year. So after their average duration of their career an NFL player will make 1.8milion dollars.
Now, lets take the average working american, with NO college degree. They average 41k in the USA for their salary. The average working American works 44 total years, and accrues benefits, and typically recieves a 20% pay raise every 4.1 years. However, for this example I am going to stay flat on career earnings for a working American. The tax bracket for these individuals is the lowest and are only taxed 10%.
So with that, an average american would make 1.6million dollars over their career not including any pay raises, career transitions, benefits, or retirement. If you add a degree (which all nfl players have) then the average working american will make over 2 million dollars during their career.
There is a false belief that NFL players make more money during their careers than successful working americans. Its simply not true. The NFL is a cruel place for these players, and the NFLPA needs to do more.
So your comment about "there is nothing else they can do to make that kind of money".. its wrong. Having a stable career in one of the hundreds of fields that net you decent to above average money, will have you much better off than the average NFL player over the long term.
Bonus note: My company is paying kids out of college at a starting salary of 80k with bonuses. If thats their starting point, then their end point will be over the 10s of millions during their careers.