The way you say that you think Nonito Donaire should retire simply shows that you think his career is over. I think let's just watch from the sideline the outcome if ever Nonito decided to move to lighter weight. I know he still has that KO punch but then I hope he still retains 90% of his speed. I think he has an advantage in the lower-weight division since I believe he can withstand punches in that division more than his current.
Yeah I think he's still can compete with the other boxers, Inoue is too much for him since Inoue currently is the top 1 pound for pound professional boxing, he even beat Usyk, Crawford, Spence, and Canelo. If Donaire will lower his weight, then he and Jerwin Ancajas will start collecting a belt for their country. If Ancajas is challenging Martinez, then Donaire could challenge Joshua Franco.
Many boxers at same age will decide to retire. Donaire at 39 with 49 fights in his pro career, its a lot. He had a good run no doubt about that. time has passed.
At certain age, if you yourself will know its time. Its not like other career that doesn't need agility of your youth. Your skills doesn't depend on how fast you chop the spices but how good the taste of the recipe.