The day a monkey decides to bring it's own lawsuit
Just to work out what your criteria are here, and as a thought experiment, what if we replace your notional "monkey" with the counterexample of an adult human with severe learning difficulties? Such a person might also not be able to bring their own lawsuit, or go on the witness stand in a way useful to their case. Do you consider that they would (should) have any photographic rights in this kind of `selfie' case?
Is "bring it's/their own lawsuit" and "go on the witness stand" really the only relevant criteria?
Or do they have to be a member of a species who are usually capable of bringing lawsuits, even if the one in question has a significant disability that impairs their ability to do so? Worse, even many ordinary humans have a poor understanding of their own local law (let alone that in other countries), and frequently/usually have to rely specialist assistance in bringing lawsuits. Sometimes they even get coaching so that they can act as an effective witness.
I'm not trying to argue either way here. I'm just trying to locate the kind of grey area which can be helpful in clarifying exactly what criteria should (at least arguably) be used.
Perhaps the question should be: even if most of us agree that this monkey in this case doesn't/shouldn't have any photography rights, what sort of hypothetical monkey is required to /only just/ pass our test for being granted these rights?