Way to simple, and misses a lot via the simplicity. Lets assume a UA 789 which costs $15,000/hour to operate (UA's cost when they first flew them, just going with it...). 12 hour flight. So Operating costs are $180,000. 48 J, 204 Y.

Assume the following:

Discount J 30 x $2000
non-discount J 12 x $5000
TOD upgrades 3 x $500

discount Y 154 x $500
non-discount Y 50 x $800
Total = $238,500 this is enough to be profitable.

But assume, less "discount" J demand (10 less seats) and less discount Y demand (30 less seats) then the math is:

Discount J 20 x $2000
non-discount J 12 x $5000
TOD upgrades 6 x $500
Discount Y 124 x $500
Non-discount Y 50 x $500
Total = $205,000. This is probably no profit, even a loss given overhead.

Now this does not assume that UA discounted yet further (which would cut into this even more) or they held open the discount prices for longer (which would cut into the non-discounted fares) and assumes that the "non-discount" travel - much of which is elite travel - stays the same.

Again, every part of the plane contributes, and the key passengers are those who will pay more than the average price (i.e. non-discount, usually later purchase business travelers) in all cabins. BUT, and this is what this example is designed to show, even the discount traveler is important. Have too bad of a rep, have some of this traffic skip you, and it will kill the bottom line just as easily.

This is why Elite programs have been so important in the past, and why United is so stupid to be shafting the middle ranks of it's elites with such high (actually punitive) levels of spending to qualify for elite status.