Yes, I have done the same kind of math. My 1st, 2nd, 3rd, and 4th cars are all Porsches. All bought used. All bought cheap. All maintained at low cost by me personally. So every day I drive a Porsche to work. Which one I pick depends on the weather. Each car gets 3K to 5K miles per year. Each car gets driven regularly, except I'll pull one and put it in storage for ~3 months just to keep my insurance costs down and use that opportunity to make any upgrades. Really my biggest cost is insurance! The good thing is that my wife is an accountant and understands the math and supports me in my Porsche hoarding. I've got two more cars that I am restoring that will be added into the rotation. I plan to keep all of my cars for 20+ years, and with the miles I'm putting on them I believe that's a reasonable expectation without major rebuild costs. Also, when I do finally sell off some of my cars I don't plan on taking a loss at all, which is what you are figuring. If you are smart in your buying and can work out some horse trading you can restore a car (even a 911) very cheap and still do a quality job. I have a 1986 Carrera in the rotation now that I have less than $10K total invested in. The 1975 911S that I am restoring will cost me a total of $4K out of pocket when I am done - with a newly rebuilt 2.7L engine, new interior, new Fuch wheels, and new paint! I do not plan on selling that car for $4K though in 20 years!
So, yes, I do believe it is quite reasonable to have a nice exotic Porsche for 20 years and to own it affordably. But I would go further to say that it doesn't just need to be your 3rd car. You could also have Porsches as your 1st and 2nd cars, and 4th, and 5th, and 6th....
Kirk