Just some additional thoughts to add to the great points already made:
- The region you are traveling in matters quite a bit. The EU has very clear, strict, and enforceable penalties for delays and cancellations so the airlines have an incentive to address the problem ASAP to avoid paying these penalties
- Flights are extremely full across the board right now -- and in particular for flights to Asia there is a very limited number of flights per day so it's conceivable that without a passenger cancellation there are no open seats for a couple of days
- It is somewhat surprising that UA didn't work to get you to TYO which would've given you more flexibility to get back to the US given the number of flights a day they have to NRT/HND not to mention the fact they are close partners with NH and also operate in a revenue-neutral manner so it truly is a wash for them to move you from the UA flight to an NH flight
- If you were booked in PE or J, you can sometimes find yourself in a situation where they can get you on a plane sooner but they have to downgrade you to a lower class of service (and refund the difference) at which point you have a choice between getting to your destination closer to the planned time (but give up your PE/J seat) or waiting at the origin for an extra couple of days but getting the PE/J seat you paid for