Detroit is an incredibly bad situation, I don't even think Americans really understand how bad it is. The whole midwest has been hurt by the move away from factory type work but Detroit's the worst.

And you really can't compare the US to England as far as this goes. There are so many different mitigating circumstances. The size of the country is obviously one, the amount of fighters is another (both of these contribute to people (casual fans I mean) in Detroit not really caring about fighters from other regions), and of course when you really get down to it boxing is just a niche sport here.

where does boxing rank among sports in the UK? I get the sense that there's just the one overwhelming favorite in soccer and then everything else is kind of in another tier. Is boxing number two? Rugby?

Here even though pro football is huge and a clear number one it seems like there are more big sports with no equivalent to the overall dominance that soccer/football has in the UK and boxing is way down the line

Pro football
College football
Pro basketball
Pro baseball
College basketball
Nascar
UFC
hockey
boxing
soccer

only the bottom ones are hard to rank.