This past season, anything over 2500 would be a big crowd.
Asking what happened to Miami attendance is like asking the chicken or egg question. Average attendance at Miami has declined every year but one since 1999 (the sweet 16 year). I remember going to games during and before '99 when it wasn't a bad venue to watch a game. Millett can seat about 9,000 without the tarps and curtains. Yeah, it's dark, and some of the seats are far away (the arena is best suited for concerts and graduations), but the concourse is vibrant, the concessions are good, the bathrooms are good, and the seats are comfy. The tarps and curtains make it feel crippled.
I wonder if the hockey team's national spotlight has anything to do with it. Like, if someone was going to devote time and money towards a Miami sport during the winter, they're going to pick hockey over basketball.