That's true on the one hand.
On the other hand they had deliveries going out, which presumably required the kitchen, which was not "shutting". There were other customers eating in who appeared to have just got their food.
Given we were an hour and a half ahead of the publicised closing time, not "half an hour before closing time", it did not cross our minds to call ahead to book.
In any case we had been out all day, our journey home involved two trains and two buses, and we didn't know what time we would be back.
Plus if a business "reacts to demand", it should say that its opening times may vary. Monkatsu clearly says on its website that it is open from 5-10 on Sundays to eat in or take away.
Eating out anywhere is a "first world" thing, and yes there might have been alternatives, but we had chosen to eat at Monkatsu!
It's not as if we had got there at 5 to 10 and expected to be served!
My post was to ask if anyone else had experienced the same thing. I wasn't expecting to be attacked for living in the first world and also apparently for having unrealistic expectations that information on a business website would be correct 🙄