As someone that grew up in the Denver area, I'd have to agree with your points. To me, Denver as a 4-sport city isn't on the same par with Boston, NYC, Detroit, and Chicago in terms of fans. Lots of transplants to the area that have their old teams that they still root for. Plus, the teams in those other cities have been around for much longer (the Broncos and Patriots started at the same time, but the Broncos are Denver's oldest team, while the Pats are the Boston area's newest team).Of course your points still stand, because the only examples that "count" are those that support your premise. Any sports franchise relocation that doesn't fit your narrative can be explained away, simply because you say so.
Denver, where I live, is almost exactly the same size as St. Louis. But Denver has all four major sports plus MLS soccer, while St. Louis has only the Cardinals and Blues. Does that mean Denver is twice as supportive of professional sports as St. Louis? I don't really think so, but if I tried really hard and squealed loudly enough, I could shape a self-serving argument to that effect. And come to think of it, St. Louis must be a terrible sports town because the Rams went back to god-forsaken LA and they still haven't replaced the NBA Hawks or the MLB Browns. Pathetic sports fans, those people in St. Louis.
I've been to Rockies games against the Yankees and Cardinals when Coors Field was probably half Yankee or Card fans.