In my experience, most invocations of Israel's right to exist are not functionally asking people to extend some generally accepted principle to Israel, but rather demanding that they accept Israel's existence as axiomatic. My response is usually something along the lines of "I'm not sure any state has a right to exist, or that it's even a meaningful concept to apply to political entities rather than people".

That's often not productive in terms of continuing the conversation, but I'm not trying to be clever or evasive; I genuinely don't understand why I should accept the premise of the question.