I am not, actually. I specifically pointed out improv as having a structure and building off itself so as to surprise the participants. I separated that from play acting to specifically move it away from performance!
D&D is not all RPGs. I made that clear as well -- that there are RPGs that support improv better than D&D, which is, at best, ambivalent towards it (I think it cuts against it, frankly, with the almost required need for either GM prep or massive GM system mastery or just ignoring the rules to make improv work). I'm not selling RPGs short. I'm not even selling D&D short -- it was never meant to do improv, it does something else rather intentionally! I'm rather happy with than intentionality.
If D&D actually made a move to support play that is more improvisational, it would be fairly controversial, given how many of the products for D&D could not be created (adventure books, setting book would look different, etc.). The last time D&D did this, it was rather a stink -- you may recall 4e? 4e was trivially drifted into a story now approach to play, which is much more improvisational that trad D&D play. I expect a fair amount of argument about that.