Sure, they're not Rays bad, but I don't know if I could call the Expos' history a "long history of supporting baseball." The late-70s to early 80s seem to be their only real competitive run besides 1994 and those were basically also their only decent attendance figures (unsurprisingly).
The Rays also average 14k a game, not that much different than what the Expos did when they were bad, if not better than what the Expos did. It took the Expos 10 years to average more than 20k a game and their first year in team history they were below-average in attendance (you'd think there'd be a new-team bump) and they were also below-average when opening the new stadium in 1977.
If Montreal came back and got a full team I don't know if they'd be a reliable 25k per game attendance team, which would be in the top half of the league. There's enough questions around that IMO that if I was Manfred I'd tell Montreal to prove that they could out-perform cities like Nashville, Charlotte or Portland. What's the point of adding another team that has similar attendance as Pittsburgh or Oakland?
