You're ignoring opportunity cost (and/or financing cost), registration/taxes, and insurance. It's also unreasonable to assume the vehicles will be worth $0 at 200k miles. If you assume you can sell them for even just $1k then the annual depreciation loss comes to $875 vs $800, half the difference you're stating. On the other hand there are additional repair/maintenance costs on the used vehicle but it's very unlikely for those to overcome the additional opportunity cost.
I would also like to remind you the OP of this thread has only $8k and less than $250/month of extra cash flow. Putting all of that towards a car seems ill-advised to me. I don't disagree that a new car that you keep for a couple decades can make a lot of sense but I don't think it is the answer for this particular person.
I'm not ignoring those costs.
Remember, the OP would have to buy 2.5 used cars vs. just 1 new car during this time period.
So they would incur 2.5x the registration/DMV fees, dealer fees, taxes, ect. This makes the used vehicles even more expensive than the new vehicle.
I've not found insurance to be dramatically different on my old vs new vehicles.
I'm currently paying $450 PER YEAR for full coverage on TWO nearly new vehicles.
By your own resale numbers, the used car still makes no sense, as it costs more per year than the new car.
You're paying MORE for the privilege of driving second-hand cars with 120K+ miles, no warranty, more maintenance/repair costs, and higher transaction costs.
Besides, a one-owner car with complete documentation/history, is going to have a higher resale value than than a multiple-owner car with unknown/incomplete maintenance/driving history.
I do agree with you that this approach generally only makes sense if you are financially stable and paying cash.
However, with ultra low 0%-1.9% new car financing rates, buying new can still save you money over used, even if you aren't paying cash.
I also don't buy the "opportunity cost" argument.
This assumes that every available penny someone has is invested in the stock market. The additional $9K needed to buy the new car would simply come from the pool of funds I keep on hand to pay for life...if I need to replace my roof, an appliance goes out, kids need braces, to take vacations, buy a car, ect.