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.