Not at all, you must prove that he had talent; the burden of proof is on you. I already presented my evidence, his art works which are nowhere the complexity or require the same skill that a Caravaggio would require. Now, show me a work by Rothko that has the same complexity and requires the same skill.
No, there's no burden on me at all because I'm not making any such contention. Pointing out that you've made assertions that lack any evidentiary basis is not the same thing as affirmatively contending the contrapositive of your statement.
In any event, your logic is faulty. The fact that someone who made a conscious, intellectually consistent decision to turn away from the more "complex" works of the past did not produce the type of work he deliberately eschewed is not probative of his ability or inability to produce such work. When your premise doesn't lead to your conclusion, the putative validity of your premise is irrelevant.