It's all good and well to try and convince folks that what you believe is correct and you'd like them to be on board with you, but in the end, you pretty much might have to state your case, let them state theirs, and let the audience decide which to accept. I don't see a whole lot of middle ground.
It's kind of like this: For the PP dislikers, their experience is that horses trained by PP followers tend to be resentful, or zoned out ., and the humans tend to be more likely to play games on the ground than ride. This is their experience, ( and happens to be a moderately large share of my experience, too). But, it's just their experience.
PP folks experience all kinds of joy and excitement and learning , as they find something they can really relate to. Their experience is so different that for them to hear the negative one might be almost shocking, at least baffling.
We are each allowed an opinion, it's just that the wider your experience base is, the more accurate your opinion is likely to be. I don't DO Parelli, and have met a limited # of PP folks/horses, so I will admit it's a limited experience base, so though I already have a slightly negative view, I am willing to be swayed.
If all you've ever done is Parelli training, then your experience base is limited. You can still have your opinion, but realize that it might have some maturing to do.