Salary cap prevent to spend extra money to gain advantage.
Not true. the soft cap does allow to spend more, and to pay for is by tax that goes to the other teams in the league.
I suggest that you will read more about the subject. It is an interesting issue.
With salary cap old teams will buy players with better secondaries with the same salary as players in young teams but with worse secondaries.
With which money they will do that? With the same money that creates this advantage.
By that the revenue difference will get smaller.
In addition, as soft-cap can be defined as follows:
1) Player cost - how much did the player cost for the team who bought him.
PLUS
2) The total salaries going to the team's roster.
Due to that, this team will pay agina the tax of going over the soft-cap.
And by the hard-cap will not be able to create huge difference and will need to by smarter and not only by his economic advantage.