You would've said the same thing about LH's FRA and BA's LGW service from DEN if they didn't exist.
Both of these flights have been very profitable.
BA's DEN service is the 2nd most profitable US route from LGW (IAH is #1).
LH has not reduced DEN service as a result of 9/11 while it has reduced frequency on other routes such as PHX to 3 weekly (comes back to daily in June).
The fact remains is LH and BA saw an opportunity in DEN. If it wouldn't be profitable, they wouldn't be flying here.
Same story with MX. If they didn't believe DEN-MEX would be profitable, they wouldn't have announced the route.
Airlines are not going to start a route just because of the incentives. If that's all airlines care about. Then I suppose if COS starts offering $1M incentive for LGW service, they will get it.
It's a simple fact, the reason why they don't come is because of the high landing fees in Denver.
Just a small comparison, but the landing fees in DEN are nearly twice as much as the landing fees in SFO. So you can have 2 touchdowns in SFO for almost the price of 1 touchdown in DEN.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran