I don't want to jump on a soapbox here, but anti-labor and anti-union sentiment in the US has been high for more than three decades. That I think is the biggest obstacle in the case for Marvin Miller--no one wants to be reminded that he put labor on an equal footing with ownership for the first time in a century of established Major League baseball.
MLB loves to celebrate its civil-rights accomplishments as both Jackie Robinson and Larry Doby, the first two African-American players in 20th-century MLB, are in the Hall of Fame. Strictly on numbers, though, Robinson is a borderline-yes (didn't have the longevity but had peak/dominance) and Doby falls short of the borderline; correspondingly, Robinson was voted in by the writers and Doby by the Veterans Committee. In Doby's case, his contribution goes beyond his stats--he integrated the American League. However, good luck waiting for Doby's first uniform number, 14, to be retired by MLB, let alone waiting for a movie titled 14. Speaking of which, in this year's movie 42, about Robinson's first year in the majors, we were reminded that Brooklyn Dodgers general manager Branch Rickey had pragmatic economic reasons for integrating: It brought more talented players onto the market, from which a winning team could be built.
It is that overt business aspect that I think is the roadblock for Marvin Miller. We know that business underpins baseball, makes baseball possible in the first place, but we don't want to be reminded of it. Of course, it is unavoidable, from the cost of the ticket to see a game and the cup of beer you buy when you watch it to the tax dollars allocated to subsidize the cost of a new stadium to the size of the salary the Seattle Mariners are offering to Robinson Cano for his services--which, by the way, is a direct result of the efforts of Marvin Miller and his successful efforts to secure the rights to collective bargaining and arbitration for the MLB Players Association. That is the seismic shift that occurred in baseball in the 1970s, that players are no longer the owners' chattel but active economic participants with their own leverage.
Jumping into sheer speculation here, based on the composition of the Expansion Era committee: In the "Hall of Famers" group, I would think that players Rod Carew, Carlton Fisk, Joe Morgan, and Paul Molitor would be the most sympathetic to Miller's election as their careers overlapped the critical sea change of the 1970s. The other two players, Phil Niekro and Frank Robinson, I suspect would be less inclined as they are older, although Robinson is outspoken on other issues. The two managers, Whitey Herzog and Tommy Lasorda, I would expect to side with the owners. The Executive group (Paul Beeston, Andy MacPhail, Dave Montgomery, and Jerry Reinsdorf) I would expect to oppose Miller. The Media and Historians group (Steve Hirdt, Bruce Jenkins, Jack O'Connor, and Jim Reeves) is a crapshoot--I don't know anything about them except Hirdt, who seems forward-thinking in his baseball analysis. So, not a lot of support for Miller overall.
Miller's induction could take a while. It might take the presence of more recently-elected Hall of Fame players, whose careers were more directly impacted by Miller's influence, on future Expansion Era committees to form the critical mass needed for the necessary votes. However, Miller's downward trend is not an encouraging sign.