The other question is, is having dominant teams better or worse for the sport? I like parity, but fans seem to respond more when the Sox or Yankees are on TV. What's better, a Yankees / Dodgers World Series, or Brewers / Royals?
Look at it from an owner's standpoint, and not as a fan. You're going to have a team that can never compete on the field or off the field. You are at a considerable business disadvantage, without assistance from the success of the overall league. Without any help there is no point in owning the team.
While I see your point, that actually hasn't happened, at least in MLB. "Lowly" franchises, like Tampa Bay and Florida, have made it to the World Series. Yes, the teams that spend the most -- the Yankees and Red Sox -- make the playoffs the majority of the time, but the Yankees have, what, one title in ten years? The league gets the best of both worlds: parity in terms of actual winners, while getting revenue from having its glamor teams consistently in contention.
I'm not saying that's the perfect model, but it's worth discussing. MLB is extremely successful right now, without a cap. Would a hard salary cap make things better?
Lastly, is there any league where a team / owner is "without assistance from the success of the overall league"? In MLB, there is substantial revenue sharing, meaning that the small clubs do quite well when the big clubs are making a lot of money. Additionally, the teams all share in the national television deals and all get a cut of overall merchandise sales. Perhaps most importantly, the overall success of the league drives franchise values through the roof. The life of a team owner who owns a non-contending team isn't so bad, at least under the MLB model.
I prefer a hard cap, due to competitive advantage reasons. However, I definitely understand the counter-argument, and it has some compelling points.