well, I'm no expert on aviation. but the British had De Havilland, Dutch had Fokker, Russian have Tupolev and Ilyushin. Even in the US, Boeing was not the only jetliner manufacturer, there were McDonell-Douglass and Lockheed competing in the industry.
how come today airliner industry transformed into a duopoly between Boeing and Airbus? and could it be avoided so we could see a more diverse jetliner industry?
Either Douglas/MD or Lockheed might have managed to stay on as a third contender, but it's admittedly an open question as to whether the large airplane market can support more than two competitors.
Lockheed actually might have been better placed than McDonnell Douglas to survive had a couple key things gone better for the L1011. The decision to only offer Rolls Royce engines turned out to be Lockheed *twice* shooting themselves in the head. By most measures, the L1011 was a better aircraft than the DC-10, and the market really wasn't big enough to support both. What brought about the DC-10 launch was AA's orders; AA would likely have ordered Lockheed's jet had they had a non-Rolls Royce engine on offer.
Then, Rolls Royce's bankruptcy nearly dragged down the remaining L1011 program. Absent these factors, Douglas wouldn't have built the DC-10. They'd have faded from the market, probably going out of the commercial business by the early 1980s. Lockheed would dominate the sub-747 jet market and could use that frame as a base for a twinjet variant (which was considered OTL). They could also have used Douglas' exit to build a small plane to compete with Boeing and Airbus.
Moreover, unlike McDonnell Douglas, Lockheed's extensive military business could have helped keep the company afloat.
Today, you might be in a position where Lockheed and Boeing are the dominant commercial manufacturers, but where Airbus remains a relatively strong third (if weaker than OTL) due to European financial support.