The problem is, most airlines who ordered or are considering to order the 737 MAX have not much choice. The only real competitor is Airbus. But not only does Airbus alreay have filled order books, so cannot deliver any additional orders in a reasonable time frame, but also this would move the market into a near-monopoly, something that airlines are trying to avoid strongly, as that would remove their negotiation powers with the manufacturer.

This shows, that a duopoly can be as bad as a monopoly and can degenerate into one quickly. Consequently, it should be avoided as much as possible. Merging Boeing with McDonnell Douglas was a big mistake by these events. As was, not to develop a successor to the 737 even long before the MAX development started. After 40 years, tweaking a very successfull design isn't enough any more, there should be a proper successor.

Unless some more large flaws are discovered, the 737 MAX won't be cancelled and it will be bought. The market is trapped. Airlines, who operate on 737 only will have to change their operations soon, as the MAX can be only a gap-filler now. But on short notice there is no other possible replacement available. Boeing will certainly start a replacment design soon, but that won't arrive till the late 20ies. This might of course be the opening for new aircraft companies to enter the market, e.g. Chineese companies or Japanese.