Is there really a general trend for new cars to be purchased at a non-negotiable price, with standard non-negotiable financing, and with trade-in independent of the new car purchase? I can point to some examples (including but not limited to Tesla) where at least a "no haggle" price is the case but it doesn't seem particularly common. One thing is that, while a lot of people claim to dislike negotiating, those same people want to get a real or perceived "good deal."

There is not such a trend, for new cars. Remember that new cars are mostly sold by dealers who have exclusive territories. That gives dealers the freedom to pick a business model that maximizes dealer profits.