I also have always kicked myself for not buying a 1987 Buick GNX at $36,000 even though the MSRP was only $30,000 a 20% ADP. That car would be worth at least $150,000 still today although there is almost a 100% chance I would have sold it after 2-3 years like everything else I have bought.
I am not sure if the state you live has a law that new cars must be sold through a dealership. Most states do have that law. When by law you are forced to buy from a dealership then that is the opposite of free market. The dealership by law does not have to compete against all sources only other dealerships. Take away the “by law” and then it becomes free market.
Dealerships also are notorious for playing games with sticker price and MSRP. Dealerships make lots of money selling you a car at or below MSRP no matter what they tell you.
BTW ADP is not ADM. Added Dealer Prep is for getting the car spruced up for the customer. Added Dealer Markup is when a dealership simply charges you $5-25k over MSRP because fools will pay it. The dealership does not provide anything more for that money. Once the shortage of that particular vehicle ends then you not only have depreciation but you also lose every penny of the ADM you forked over.
With the shortage of ICEs and EVs due to supply chain issues a yr ago, dealerships jumped in and extorted ADM because by law they could. The folks that paid did not buy a future collectors vehicle. Now that ICEs are plentiful those vehicles folks bought 1 yr ago are no longer worth the extra $5-25k they put into the dealers bank account.
With the advent of online ordering, dealers must change. People can see MSRP, can see when and where their car was manufactured, can see when and how it was shipped and when it gets to the dealer. They can order exactly what they want. No more being pressured to buy what’s on the lot. Dealers should carry a lot less inventory and compete using services. The age old hand waving, hocus pocus and BS from dealership salespeople must end.