I see it quite simply as two scenarios.
You can buy your way to success.
Or,
You can train your way to success.
Buying your way to success leads to almost instant results, but as a result of this, you tend to end up in a loop where you keep replacing players to maintain competitive as they tend to be older free agents, so you need to buy and sell. As you are more likely to sit close to your salary cap doing this, you make no money. So you remain stagnant and it's hard to improve.
By training, you sacrifice in the short term, but in terms of long term sustainable success, it's much more efficient. Not only will you make more money, but you can continue to improve those player as they hit their peak at 22-23, where they are just as effective as those 28-29 year old free agents. With that extra money you saved when they were 19-20, you can now buy the players you need to help you and your trainees to push for promotion to the top division.