Basically, the players that quit after one or two seasons are lazy people that don't want to put the time in and get upset when things don't go how they want it to go. Instead of trying to get better, they quit. Though some may not like the game as well. Now, like someone else said, what happens when the coach goes from D3 to D2? Gonna get killed because they have no idea how to do the more in-depth stuff, then they will quit D2, then the change will be made to D2 and so on and so on. D3, just like in RL, is a learning ground for new the new coaches that want to move up one day. Some decide to stay in D3 for whatever reason, some decide to move up. Either way, it's a learning ground. Hard to learn anything when there is nothing there for you to learn, so you are just as clueless when you get to D2.
This to me is generally correct, Div 3 is the best place to learn how the game works, there is no danger of getting fired and as you take on your recruits your team should improve to help you understand what works and what doesn't. This new "idea" will just create a division full, if they are really lucky, of coaches with little knowledge and less ability to move up. If veteran coaches are allowed to stay in D3 new players can join these divisions and learn from the coaches already there, I know from my experiences if I had questions on the coaches forum no coach was ever unwilling to help the newbies.