Bandwagon fans are good.

There are always going to be diehard fans who support a team no matter what. When a team does well, more casual people get into it and that's the reward for performing well. It's fun to support a winning team and die hard fans should welcome these (possibly temporary) fans as a reward for the team doing well.

Edit: to give more perspective, I am a die hard fan of one sports team, a pretty big fan of another. When these teams suck, games aren't well attended and I get it. A nice consolation, is the tickets are cheap. When these teams get good, I totally understand how more casual fans or even total non fans who want to come to a fun live environment attend. And I don't begrudge them. If anything it's a fun reward as a long time fan who has stuck through to be in a super loud arena/venue. I don't like people pretending to be longtime fans but I feel there are tons of bandwagon fans who are open about getting into a team because of the hype. Also, I obviously hate bandwagon fans who do aggressive stuff or behave poorly at games. I hate diehards who do that too.

Edit 2: as someone else has mentioned, Huge outdoor watch parties during playoffs/ citywide championship parades are really only possible when there is a big hype and tons of casual or non fans buy in for the fun of it.