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.
Even the DC-3 was developed in response to a request from TWA for something similar to the Boeing 247.
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