There are so many factors that go into this:
No practice squad available to develop players.
Forget 5th, 6th or 7th round picks. Whens the last time someone signed an undrafted rookie free agent and they made the team? Not as some 3rd string/special teams player but a starter. It happens all the time in the NFL...I'm a Ravens fan and we have had an undrafted rookie free agent make the roster 17 straight years.
My point is the talent pool in the draft is a known quanity...sure volatility factors in but you never have one of these...
https://www.stadiumtalk.com/s/best-undrafted-nfl-players-3300be305ccb43e8NFL teams make decisions based on potential versus compensation. An undrafted rookie or late round pick is cheap and has potential while a 5 year vet is expensive...In MFN, we are fully aware of a players ceiling post training camp so if it isn't an upgrade, we keep and pay the vet.
Its just a completely different dynamic.
As far as older free agents being available...its partly true in the NFL as well. Only players who have enough service time to receive a pension have to officially retire. The list of players that show up at try outs and training camps is astronomical. Many don't let go of there dream even if they have never made a roster and are 5 years out of college....Kurt Warner was bagging groceries before he winded up in the Hall of Fame.
Unlike the NFL, we can look at a players card/attributes and have a very good idea what he will be while the NFL skews younger due to veteran minimums and he hope the younger player might develop and become a starter.
MFN isn't identical to the NFL in many ways and building your roster here like a real NFL team will actually hurt you with all the "beat the sim" type owners who have zero problem playing half there roster under 200 pounds because lighter/faster players gives them an advantage.
With all that being said, its the best football sim I've ever played....owners that use all the exploits to gain an advantage do ruin it for everyone attempting to simulate real football so some custom leagues have adapted rules to prevent the use of exploits.
JDB could easily fix the exploit hole by severely increasing the injury risk to any player that plays a position where they are extremely underweight.