While I'm sure not everyone would agree with that, it's not exactly implausible. Indeed it must surely seem likely to anyone who has observed how wildly differently young children react to the same situation. And has been borne out in twin studies, etc.
Now that doesn't mean that the MBTI has the right theory about the nature of the "nature" component, but my point is that if you are taking the MBTI to be a description of someone's total personality rather than just the nature component of their personality, then you are completely misconstruing what it is claiming.
As I understand it that arguing nature vs. nurture usually stems from badly defined concepts often defined in terms of how they are measured (e. Operationally defined). What happens then is that you get a bunch of correlation between various concepts just because one concept pollutes into another, not because of how we are geared. Nature vs. nurture is also an awfully dualistic approach and completely misses (or ignores) interaction effects.
To tie this into this thread. GP is not saying that nurture defines personality as opposed to nature. They are simply claiming that it is malleable. How malleable, under which circumstances, malleable to what, which aspects, etc. are all still open questions which may be ascribed to ones genes (and maybe not; hence the open question). This is the interaction effect which the dualistic nature vs. nurture debate completely misses.
I would actually go further then GP and say that the question is flawed. Personality is not a useful psychological concept and does not describe behavior any better then, say, political leaning, religion, or how many languages you speak. That is to say, measuring personality and categorizing people based on personality traits brings us no closer in understanding human behavior. That is to say, measuring personalities and ascribing traits is not only harmful—like GP says—but also bad psychology.
> How malleable, under which circumstances, malleable to what, which aspects, etc. are all still open questions
Indeed. And the MBTI theory makes a specific and very interesting claim about this open question: that what is malleable, under which circumstances, etc varies between people. And furthermore, that there is an extent to which that variation follows certain set patterns (possibly as a strong statistical average with exceptions rather than a sure thing), such that we can classify people by which pattern(s) of malleability they follow.
This doesn't mean that all malleability is subject to these patterns. There can still be aspects that are common to all people, and aspects that are completely individual. But the claim is that there also some aspects that apply to group of peoples.
Of course, such a claim needs to be substantiated and the current evidence is mostly from clinical observation rather than scientific experimentation. But whether it is true or not there is good reason to believe that the lack of scientific evidence stems from a lack of ability to measure accurately because (as you say) mainstream psychology has very little to say on this topic at the moment and exactly how this aspect of the mind works is still considered an open question.
> That is to say, measuring personality and categorizing people based on personality traits brings us no closer in understanding human behavior. That is to say, measuring personalities and ascribing traits is not only harmful—like GP says—but also bad psychology.
I should note that the MBTI does NOT measure external traits. It (attempts to) measure an actual functional mechanism in the mind/brain (which it postulates to exist).
Which proof do you have that such a thing exists? Underlying personality traits sounds to me like a theory of epicycles when a simpler more plausible theory exists.