Roster change over is huge in the NFL.
IF you hit in the Draft, you could improve of course. But, now in the NFL a Drafted Rookie after is FA after 4 years, 1st Rounders are 5, if 5th year Option is picked up. Then they are FA's.
Now you start talking money, teams have to Manage the Cap. Sometimes you have to let some of your good players to leave. Just want too much. Dead Money is part of the Cap.
In the long run Days of Steelers, Cowboys, Packers, keeping a Player Drafted, for the duration of their Career, is harder now. That's why what the Patriots are doing is unreal.
Sure if you hit on the Draft, and that's a big if. You'll improve. But you need other ways to keep a good team competitive for any long stretches.
Baseball needs a Cap, in the worst way. But, it will never happen.
Salary Cap has made the NFL more competitive, more then anything.