It is just a matter of management.
If you buy a 5* fast trainer he will reach the scout level during 4 seasons, only doing training every day, then you probably can keep him for 2 seasons more, maybe more, depend of the rest of your squad.
If you buy 1 scout near the 7* star he can be in your Team the same 4 seasons, but because of age If you are not able to PT him, it will be dificult to keep is stars, so is life is Team will be shorter.