They don't have a say in the matter, but neither would the animals you propose to feed to your cat. Those animals which are almost certainly affected in a much more negative way by being raised for pet food and then killed than your cat would be by responsible trialing of the vegan food.

Not trying to be a jerk, but it bothers me quite a lot me that people use this argument so often and only consider the perspective of their pet rather than that of all affected individuals. It seems to me that rejecting a "possible and practicable" alternative which reduces exploitation and cruelty would be difficult to justify.