Life has convinced me not to be. Far too many human qualities are inborn, both strengths and frailties. It is "psycho-utopian" to believe otherwise.
I know my limits pretty well, and have always been determined to blame my disappointments on myself. That's the best, most honest and self-critical approach even when it is ego-damaging. Ego-damaging is soul fertilizer, the wise men say.
But are progressives "Blank Slaters"? Not really, no. Just selectively. This is a longish read, but worth your while: Selective Blank Slatism and Ideologically Motivated Misunderstandings.