Fans support their teams emotionally. Loyalty should run both ways, but it seems to be the fans' lone responsibility. A basketball or football team is not peanut butter.
Take the Colorado Rockies (please!)... They make money with a smattering of stars but mostly an underwhelming team. There's no hope of competing with the rest of their division for the next several years, but ownership is satisfied with the status quo because they are still making money.
I was going to say that perhaps that the biggest difference between "the old days" and now is free agency. But players have been moving from team to team since the Bambino left Boston. It is so much more widespread now in all sports that I sit an ponder my own loyalties. Why should I care if a team put together almost at random represents my town?
Human nature, then, must be at the root. Humans must have a built-in need to cheer for something or someone and this need can't easily by quashed. That makes us so much better than dogs and cats... We're number one!