This is true, but misses the point that if another airline could enter the market with a desirable product, or an existing airline could add a fast service. The airline market is a highly competitive one, as all the vendors are distributing largely undifferentiated products The airlines and manufacturers regularly collaborate in the design of new aircraft.
You are correct, and a pattern has emerged: there are the airlines that differentiates themselves on service that is tied to their country of origin (Singapore Airlines vs Emirates, both fly A380, but definitely different feel to them), and then there are no-frills airlines.
The collaboration you speak of are mostly to do with cabin sizes, how many seats, which variants to make to satisfy the majority of customers -- these can all be changed when the main parts of the plane stay the same: e.g. fuselage, engine, etc. Rarely do an airline get Airbus/Boeing to change main parts of the planes: one exception is EasyJet when they bought an insane number (200?) of A320s from Airbus: Airbus made a fuselage specially designed for EasyJet that had more seats and they had to move their doors.
If you look at almost any modern airliner's development cycle, you will see that a significant period of time was spent simply developing the specifications in collaboration with the airlines. I am not familiar with any commercial jet that has been developed without commitments from airlines to buy it.
Even the DC-3 was developed in response to a request from TWA for something similar to the Boeing 247.
It's not about satisfying the needs of one or two airlines though. BA reportedly made a profit operating Concorde in the latter years: it was a very nice niche product for them. The British and French governments lost a fortune on a programme that only saw 14 aircraft ever delivered to end customers.
Compare that with the hundreds of new commercial aircraft orders today (it's the first day of the Paris Air Show) from aircraft with a more widespread demand. The A380 is regarded as a niche aircraft with not-entirely-satisfactory sales figures, and it confirmed 20 orders today to add to an existing order book of over 260
