The only airlines that are Boeing or Airbus exclusive are low costs (like Ryanair or Spirit Airlines) and tiny airlines operating a handful of planes. Otherwise it's all mixed fleet (Delta, Air France, JAL, ANA, Lufthansa, UA, etc.). If you separate wide-body and narrow-body then your statement is more correct: as going narrow-body Airbus and wide-body Boeing (or the other way around) is not rare.
The threshold I've read on airliners.net is at about 30 frames. Below that costs of a sub-fleet are too high and you should aim for commonality. Above 30 frames you should pick the best plane for your routes, regardless of commonality.
So for example running two fleets of 30 A350-900/1000, and 30 787-9/10 would be perfectly rational. But having 50 A350-900/1000 and introducing a sub-fleet of 10 787-10 is unlikely to be a wise choice, even for routes where the 787-10 would beat the A350-900.
> The only airlines that are Boeing or Airbus exclusive are low costs (like Ryanair or Spirit Airlines) and tiny airlines operating a handful of planes.
This isn’t true for the reasons you are implying. Most of these smaller carriers fly to regional airports with smaller runways and smaller demand which the 737 specifically is ideal for (since that’s what it was designed). It’s also one of the most common planes out there, so when smaller carriers are looking to buy, that’s what’s available. It’s not that they are looking to be a single vendor airline. It’s just that the one vendor made the plane they need.
On the lower end the 737 has competition from Embraer, Bombardier (and now Airbus through the CSeries/A220), ATR, etc. Turboprops in general (and thus ATR) are not popular in the US, but they are quite popular elsewhere.
But the specific example I had in mind is Air Tahiti Nui, and its grand total of four 787-9. It’s simply an airline specialized in bringing tourists to Tahiti from far, far away. Hence high density 787-9.
But even a very small airline like Air Senegal has a pretty diversified fleet, of 9 aircrafts…
The threshold I've read on airliners.net is at about 30 frames. Below that costs of a sub-fleet are too high and you should aim for commonality. Above 30 frames you should pick the best plane for your routes, regardless of commonality.
So for example running two fleets of 30 A350-900/1000, and 30 787-9/10 would be perfectly rational. But having 50 A350-900/1000 and introducing a sub-fleet of 10 787-10 is unlikely to be a wise choice, even for routes where the 787-10 would beat the A350-900.