It isn't going to increase profits. The seats go out empty because they're overpriced; taking away upgrades isn't going to change the market's opinion and is also going to cause space in the back to be occupied that might otherwise be sold. I fly DL from JFK to SEA because it's cheap and there aren't a lot of alternatives, but LAX and SFO flyers apparently have a lot more options and are going to work the unavailability of upgrades into their choice of carrier. So, in addition to empty seats in the front there are likely to be more empty seats in the back.