First-class air travel
From an airline perspective there are many reasons why the first class does not make economical sense.
For me personally one stands out - if you look at the services that come with the first class i.e. a spacious private room in an increasingly cramped cabin, top of the line interiors, shower, more crew and a top notch chef are just minimum requirements. This means every time first class seats are not filled the airline is simply carrying dead weight and a lot of it, which could otherwise be used to sell at least 3-4 economy seats for every trip. So in addition to the loss in revenue it also incurs additional cost for every trip for added services and for fuel too since the aircraft would need more fuel to carry more weight.
So unless you have plenty of frequent first class flyers who fly all through out the year during all seasons - peak or lean, the math just wouldn't add up.
International first class is becoming scarcer as airlines focus on upgrading business class (e.g. Delta One, Virgin Atlantic, British Airways), yet Emirates is upgrading first class. Domestic (U.S.) airlines seem to be focusing on premium economy (e.g. Delta Premium Select).
Also, first class pricing is becoming cheaper, as airlines look to receive some revenue versus giving away first-class seats to upgrades. I've found this is best where you "do not" have status. For example, on the rare occasion I travel on an airline other than Delta, I look at pricing holistically on top of my ticket price: $25-40 for checked luggage, $15-20 for priority boarding, $50-60 for premium economy, $20-40 for drinks and food, etc. In reviewing pricing this way, I'm often surprised how close first-class fares are to economy. On transcontinental U.S. flights or flights to Canada, I consider upgrading. It's worth investigating.