Here's explanation of my pro-ultra-behaviorist position.

First, I haven't seen any convincing evidence against ultra-behaviorism, but plenty against ultra-innatism. Look at Flynn effect for example. There's absolutely no way a universe in which ultra-innatism is true is compatible with Flynn effect. There has been so many drastic shifts in behavior without slightest shift in underlying genetic makeup of population - abandonment of violence, shift from large families and low offspring investment to small families and high offspring investment, shift from agricultural to urban lifestyle etc. - these are vastly greater than any of the proposed genetic variations. And not a single kind of proposed genetically-based behavioral variation had a convincing genetic marker found for it (yes, there are heredity studies on twins etc. but I find they highly unconvincing). So my estimate of the truth is far closer to ultra-behaviorist end than ultra-innatist end, so much closer than ultra-behaviorism might be a good "tl;dr" version, even if not 100% accurate.

And second, I find ultra-behaviorism instrumentally useful. Overestimating how much you can change your life leads to better outcomes than underestimating it and just giving up.