One of the things I always appreciated about Vincent and the Doctor was that it showed how real depression could be, and how it haunts you day by day. Despite all of the adventures and all of his great works, Vincent still had with him the feeling that he was never enough, and that while the entire thing with creature, it is only the one bright spot in a history of lows, even then marred with the true purpose of the creature. That feeling of emptiness and hopelessness is what you remember more, no matter how well you paint or write or do anything because that is all your brain taught you to know.
That being said, to introduce that concept into a family show is amazing in helping people understand how it would affect even the greatest of men, let alone us. And the final scene, how they celebrated his life and his works at the same time accepting that both good things and bad can come from our minds, and that it is ok and it is a part of life, that I think is the message worth telling, especially for sufferers and their carers alike.