The iPhone XS was the exact same price as the X and that sold plenty fine. It wasn't the price as his letter points out, much to the dismay of many people.
Or that might not be the reason, I don't know enough to analyze the situation by far. I just don't think we should rule out the extreme price increase as a potential partial explanation.
But the people I know who switched from 6S / 7 are very much happy with their decision to buy an XS.