Miller inherited the same problems Gene Upshaw inherited in the NFL, bot changed it. Upshaw for the betterment of the league, Miller for the detriment. Yes, both men improved the compensation for the players immensly, but Miller helped to create a system that rewarded small market teams for not spending anything and allowed big market teams to buy championships. It was a huge portion of his doing.
Gene Upshaw was very instrumental in pushing the owners to come up with a revenue sharing model. He knew that without the big market teams sharing parts of their revenues with small market teams, the players would not get all of what is coming to him. From the mid-80s to his death, he was very much into pushing for revenue sharing. Miller didn't share that same forsight. Granted Upshaw did have the advantage of seeing the mess that MLB baseball had become with no cap or revenue sharing.
The general concensus is that Miller did screw up a lot of what is wrong with MLB today. He even today doesn't understand what a salary cap means to the future prosperity of the NFL. Here is what he said in February:
Marvin Miller Says NFL's `Company Union' Needs to Play Offense in Talks - Bloomberg
He doesn't get that a salary cap promotes parity which is what makes the NFL a national sport and why MLB is regionalized.
Miller is very much a main contributor to what MLB is today. He had no interest in trying to create a level playing field for teams throughout MLB. He wanted a free market system with no rules. Upshaw was smarter than him in this matter and he knew that allowing Jerry Jones and Daniel Snyder to have $400 million payrolls doesn't help his players as a whole if the Glazier family and Mike Brown spending $30 million on payroll.
I'm not sure where you're getting some of your information. The NFL was managing + selling its rights collectively and sharing revenues since 1961, before Upshaw was even a player in the NFL, let alone a union rep. If anyone deserves to be singled out for credit for the NFL's adoption of its revenue sharing model and centrally organized structure, its Pete Rozelle. During Upshaw's tenure as union rep, the NFL actually saw its revenue sharing model weakened significantly when, in the mid '90s, Jerry Jones sued the league and leveraged a settlement allowing franchises to enter into individual deals regarding name rights and endorsements, and local media. Obviously, Upshaw had little if any role in this.
The same can't be said for the 2006 CBA, in which Upshaw successfully fought for the
players' to share in revenues from local rights management, while the revenue sharing agreement among the ownership was only modestly -- and in retrospect, insufficiently -- adjusted. That said, Upshaw's responsibility was to his union, and securing the players a share in local NFL revenue streams was rightfully his #1 priority.
So clearly, while Miller and Upshaw were both brought in at similarly crucial times for their nascent unions, the leagues they were negotiating with could not have been more systemically different. MLB had already calcified in its regionally operated and managed organizational structure when Miller became involved in the mid '60s, and the NFL already had a much more robust central league operating authority when Upshaw became union head. Both of them successfully fought for their constituents' free agency, within the parameters of the organizations they were negotiating with.
Miller isn't responsible for the absence of a salary cap in baseball, because MLB lacked the central authority to negotiate one. With only negligible revenue sharing in place and an already stratified field of franchises, there was no incentive the league could give to get the big market franchises to agree to one, and no central resources to subsidize the small market franchises to meet a salary floor. Miller had nothing to do with creating the systemic inequity between the small and large market teams -- that had long been in place.
While increasing the cost of payroll by removing the artifical restraints on players' market values did, of course, constrain the small market teams more than the large market ones, this is akin to Upshaw's securing the players' stake in local revenues. Both were moves that the unions had to make, trusting that the other side would handle the resulting internal issues. The NFL has proven capable of making those adaptations. MLB has not.
So I don't know who holds to this conventional wisdom on role of Marvin Miller you refer to, but I'd be interested in hearing any of its adherents make a cogent argument why Miller is responsible for baseball's problems, especially considering that parity in baseball has actually significantly increased since his tenure, even if he didn't have much to do with that. I also don't think you can really point to parity as a primary factor in the NFL's rise to prominence over baseball. Seems to me it has more to do with the NFL's ability to collectively negotiate TV deals that promote the growth of the league as a whole, as well as fundamental changes in the way we consume sports.
Baseball was kind back when following a sports team meant for most people listening to games on the radio and/or reading about them in the paper the next day. Going to the game was only way you could see it being played, and the majority of fans could only do this on occasion -- but at least baseball offered you 100+ chances over the course of a summer. So doesn't it make sense that the sport that can be more accurately understood from a box score and more closely followed on the radio would gradually lose out to a sport that's much more exciting to see played as watching games on TV became the primary media for following your team?