Additional thought -- regardless of the number of dollars shelled out, the purpose of First Class is that it's supposed to be a "nice" place to be on the aircraft. That includes a bigger seat, a higher F/A to passenger ratio, higher quality food, etc. Dressing well makes First Class really feel like it's a nicer place to be on the aircraft (just my opinion, of course).

Look at it this way -- why go to a five star restaurant, when you can get basically the same food at a tenth the cost by going to a fast food restaurant? If a five star restaurant seated people in tank tops and flip-flops, it would't "feel" like five stars.