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>both Boeing and Airbus have given up on quadjets.

It is possible “to make appropriate decisions” up to a certain size. They didn’t stop making new quadjets because the design doesn’t work as well as a twin engine, but because airlines don’t need/want aircraft that large. You wouldn’t build a successor to the A380 as a twin engine.

[loeg]

Yes, and? If no one wants them, it's a commercial dead end.

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Airlines currently don't want them (which is not even 100% accurate since airlines pulled A380s out of storage, and continue to push back plans to retire them). You started this by saying "You know you can just make the wing engines 50% more powerful, right?". You weren't talking about commercial decisions, you were talking about engineering decisions and capabilities. So, no you can't just make twin engines bigger in all situations. If airlines want large capacity aircraft again, they will be quad jets, not super powerful twin engines.