There is not sufficient demand for airframes in the world for both the major manufacturers to supply the full range across, say A318~380. We can already see that the US has combined (major) commercial airliner development into a single company, as have Europe.
There are then a number of small companies such as Embraer and DHC for the turbo props and commuter stuff, as well as the Biz jets. The Russians and the Chinese each have their companies.
Once Airbus had decided to make the 380 the only commercial decision left for Boeing was to NOT to make a competitor. They went for the 78 and a further extension to the life of the 74, which was very sensible.
Therefore ... everyone has won! Your question is answered!