But if the airline counts too much on those fees for profitability, they’re going to get in trouble.

Agreed. We never worried much of scheduling, as albaby did. Most of our flights are on the long side, whenever we fly. So, we want premium economy (or better...but usually we end up with premium economy). We need more luggage than an underseat bag (I can usually get by with a single carry-on, plus my underseat bag which is my camera bag/kit). Longer flights, we need food. Etc. At that point, there is no advantage to Spirit.

As you say, if an airline relies on the various up-charges to generate profits, and its passengers are not indulging in those up-charges, they are going to have revenue problems. Which, apparently, they did.

As for Detroit, they have had big problems for decades. To put it simplistically, they don't make what people want. Going back in time, I remember when "made in Japan" was a joke, denoting bad quality (much as "made in China" is today). But then the Japanese cars got really good (there's a documentary about the American guy who was responsible for Japan kicking our butts...forget the guy's name). We were producing rubbish...unreliable rattle-traps**. I think we've most fixed that bit, in part because a Ford may very well have a Toyota engine and a VW transmission. Based on ads I view, US companies mostly emphasize trucks (including monster SUVs like Yukons). The ads for cars are Audis, VWs, Subarus, etc. I see a fair number of trucks on the road, but it is dwarfed by the number of sedans. And most of those sedans are either Japanese, Korean, or German.

**Anecdote: when I worked at the observatory, the town car we used to shuttle between the university and the base camp was an AMC Concord. It got crappy mileage, the A/C was AWESOME (which, in Tucson, is nice), and when you hit about 55mph on the freeway the dashboard would start to vibrate so violently you literally couldn't read the speedometer with any precision (an analog needle).