After the 2000 census, it doesn't even show up among the top 20 and I believe it had fallen to #25.
That's a very interesting table. Buffalo was the 8th-largest city in the U.S. in 1900, and was ranked among the top 20 as recently as 1960. And yet it's never had a big-league baseball team since the Bisons folded in the 1880s.
Regardless of whether St. Louis or Kansas City was the westernmost team before 1958, the point is that there was a whole lot of real estate in the western U.S. with no major league teams back then before 1958. No Houston (#7 among U.S. cities in 1960), no Dallas (#14), no Phoenix, no Minneapolis/St. Paul, no Seattle, no Denver, etc. In fact, there were no teams at all in the Mountain Time Zone when the Dodgers and Giants moved out to California.
It's worth noting that even to this day, Milwaukee only has two big-league sports teams (the Brewers and Bucks). It simply isn't much of a big-league sports city, in terms of either size or a long tradition (like Cincinnati has). In that respect it's very similar to Pittsburgh: a city in decline that struggles to hold onto its professional sports franchises as other cities become better markets.
It's worth noting that even to this day, Milwaukee has only two big-league sports teams.
Well, you could certainly consider the Green Bay Packers as a Milwaukee home team. At one time, half the Packers home games were in Milwaukee County Stadium, same place where the Braves and later the Brewers played.
Please, let's not concoct any more excuses for Selig's subpar record as Brewers owner. (You never got into his history as caretaker owner of the Expos, I noticed.) The fact is that his poor history as as an owner at least somewhat undermines his credibility to intervene with a current owner, kind of like the proverbial pot calling the kettle black.