Baseball tradition is rarely as traditional as we think it is. There are so many variables in the game (the DH, park sizes, mound, ball, length of season). Including this. And while the current analytics and usage trends are new, the overall trend of carrying more relievers and wanting a bigger bench isn't (in the DH/LOOGY era, at least).
The question for the MLBPA is simply one of money, I'd think. Do they want to create one more major league job/salary or fight for a larger share divided among the current roster size?
The 25-man roster was first created officially in 1920; before that, roster sizes varied from 16 to a limit of 21. Most teams carried small pitching staffs, as starters frequently threw complete games. The 25-man limit was in effect only from May 15 to September 1; many major league clubs would carry as many as five extra players back from spring training during the 1940s and 1950s. The minor leagues began complaining that their rosters were unstable in the early season, so in 1957 a 28-man limit was set; teams could still carry three "extra" players until May 15. In 1968, the May 15 date was eliminated and the 25-man roster limit set from the beginning of the season.
There it has stayed since, with a couple of exceptions: during a period of labor dispute from 1986-1989, the collective bargaining agreement provided for a "minimum" of 24 players, and teams cut their rosters accordingly, seemingly out of spite (they had also done this for half the season in 1978, until July 1). When MLB's season was delayed due to labor stoppages in 1990 and 1995, expanded rosters (27 players in 1990, 28 in 1995) were permitted for a short time when the season began.
But other than that, it's been 25 men from Opening Day until September 1 since 1968.