Never for safety reasons. Twinjets are allowed because all of them can stay airborne on one engine and reliability is good enough to assume both won't go out at once. There is no technical reason that one engine cannot make most small and some medium sized airliners take off by itself (obviously in a different position, maybe in the tail?) from a thrust perspective. The 777X engines have 105,000 lbf of thrust EACH. Each 737 engine, for example, has less than 30,000 lbf of thrust.I wonder how long before we see a single engined jetliner.
But one of the benefits of the smaller two engine planes is that they're cheap enough and low-risk enough that they can service secondary and tertiary hubs. If anything, knee-capping the primary hubs will just strengthen the position of the smaller planes.Have the world's biggest hubs (eg Heathrow, Schipol, JFK, LAX, Qatar, Dubai, Singapore, Hong Kong) suffer from even more congestion, and raise the per plane landing fee in response. So, only bigger planes are economic.