NFL teams with great coaches are the only ones who are successful, and coaches are not subject to the salary cap. Stud QBs have never been bought by the highest bidder. They've always been zealously guarded by whoever drafted/originally signed them, to the point where a big part of the NFL's stance against free agency until relatively recently was due to "protecting" QBs from poaching. NFL team spending has always been in a rough parity as well. There's just not a lot to indicate that the salary cap has been the driving force in any change to the competitive landscape there.
OKC is not a dynasty yet, and this is really their first glimpse of sustained success in franchise history, even going back to the Seattle days. If we're talking NBA market size, four of the seven dynasties (Lakers, Bulls, Warriors, Celtics) since the cap started are large markets and account for 66.7% of dynasty championships and 55% of overall championships. I don't see how four teams accounting for 55% of championships in the last 40 years shows a salary cap win.
I've also already outlined why I think a cap and floor won't work in MLB in a previous post, but it really breaks down to A) The cap isn't going to be below the current highest spend, B) The high spenders will still spend the most, and C) Floors are only going to enforce bad decisions from the poorly run teams at the bottom, and you can't force them to pay more than they are able to. A better revenue sharing model and standardized TV rights are much better ideas.