Nonito's career is not over, but if I would suggest, I think he should retire already.
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.
With the experience he have in the industry and the talent he possess, I can say that Donaire will be deadly in the lower division but the catch is that we must accept that he isn't the Filipino Flash that we used to know anymore as he's already aging but the power is still there, I don't doubt it. But yes, you are right these are just mere speculations and we should watch him first before saying that he should already retire.