A competitive league is better than 4 or 5 teams on top each year, unless you are one of those top teams.
I agree the success of the PL includes the rich history of many of the clubs, but rich history doesn't attract foreign players, or global fans.
The next generation of global fans won't be as emotionally involved in PL clubs as the current generation is. It would only ever be a slow shift, but I would expect a salary cap in the PL would change the attractiveness of its clubs to the best players in the world.
Does it matter? Well, maybe it shouldn't, but with PL clubs having tasted the riches then its unlikely enough clubs would agree to such a cap.