Forget about understanding subtle distinctions – at the very least, let’s be clear what the issues are, and avoid the straw man.
There are many claims mentioned in the letters, such as misquoting and attributing extra details, that are not benig addressed here at all.
But to stick to what IS being discussed –
No one has a problem with learning different Peshatim than Rashi, whether “pshat” or otherwise.
We learn Chumash Rashi. NO ONE ever learns Chumash/other pirush, disregarding Rashi. You might have learned Rashi in oast, and are now learning Ramban etc, but not to say, I’m learning Cumash/Rashbam, it’s the mehalech I use to understanbd Chumash. No one has done that. Therefore, to have a pirush that is being used as a basic explanation of the test disregard Rashi is a serious change.
Additionally, this pirush is being used by many to supplant Rashi. Many people are learning Chumash/pshuto shel mikra. That is a fact in many circles. This alone is a problem – Artscroll does not imagine that someone will use their pirush as “the” way they understand Chumash, as the main way to learn it, at the expense of leanring Rashi (unless they are incapable of reading the Rashi text, which is a different story). The placement on the page is not the problem per se, but illustrative of this issue. It also makes the earlier issue a bigger problem – if this is becoming “THE” pshat used, it cannot just offer any pshat they choose, especially disregarding Rashi.
Let’s remember that things that are overtly ‘treif’ are not nearly so dangerous to us, and often do not even require a harsh response, if any at all. It is precisely those things whose problems are more subtle that carry the danger of going undetected and creating negative change, and those require a strong response from those whom we trust and look to for guidance.