The irony is that many of us P85D purchasers would have gladly voluntarily spent the extra money on the 21"s if there were a chance they could survive more than a few months on our terrible Eastern Seaboard roads. For some of us, it's not even the costs of the inevitable tire replacements, it's being stranded somewhere for hours while you wait for a flatbed to show up to tow your car to the nearest Tesla facility.
Since the Model S does not come with a spare tire and a thin sidewall tire mounted on a 21" wheel is prone to the type of catastrophic failure that cannot be temporarily fixed with a compressor and sealant when it impacts a deep pothole, this decision by Tesla borders on negligence.
Just to post "the other side of the coin" here, I've had the 21" rims, and live in the NYC metro area and have put on over 14,000 miles on my car in 1.5 years. I've done round trip trips to Chicago, Columbus OH, Burlington VT, Boston, and dozens of trips to the Phila and DC areas... not to mention the rest of the miles in and around where I live.
While I am extremely careful to avoid big potholes, I have hit a few. In Chester, NY, I hit a massive pothole with a fully loaded car, and the car even bottomed out (it was dusk/dark out, and my passengers were distracting me from watching the road carefully). I thought for sure the wheel and tire was going to be toast. I've hit smaller potholes and destroyed rims/tires in other cars, so I know what it's like. But when we got to our destination, there was *no damage* to either the front or rear rims. No bends, no snakebites on the tires, nothing. I still don't know why they escaped any damage, but the 21" rims aren't as bad as everyone makes them out to be. The can and do survive "more than a few months on Eastern Seaboard roads".
Yes, in the winter, I do swap on 19" rims with A/S tires, but that's just for the bulk of the cold/snow season. The rest of the time, it's 21" rims all the way.