The probelm is still the same at DEN: you have some payload restrictions on outbound legs because of the altittude and the hoet temperature in summer.
However, as suggested Mr. Webb, Mayor of Denver, the first flights should "bring the rest".
Therefore it seems necessary to have a service to Paris and London, be it by UAL or Air France and BA.
When United Airlines announced they would build additional gates at DEN for international flights, it was more or less suggested that Paris would be included.
And as far as Air France in concerned, the reports of the Denver Post have suggested Air France has actually Denver as a US destination on its list, but other routes have a higher priority.