Just a point: For that car and what you apparently used it for, a car co-op wouldn't work in any case. I doubt zipcar lets you take their cars to track days, even if they had a car you'd want to take. And at 26k miles/yr, I don't think a co-op would work either.
Rather than "trivially" making it to $1500/month, I think you pushed most parameters to the expensive side.
As a contrast, I have a 2001 Passat that I've owned for 6 years and do all the maintenance on myself. Over 6 years I'd estimate it's cost me $25k total, which ends up being $350/month. But then I've only driven 40k over those 6 years, too.
I wasn't looking at it from a co-op or car sharing perspective. I was looking at it from a "how can a car cost $1000 a month perspective". The Mitsubishi Lancer Evolution and Subaru WRX STis are probably the best known examples of post-purchase sticker shock considering a transaction cost that is only 110-115% of that of the "average new car transaction cost". Especially the Mitsubishi, which was referred to by Jeremy Clarkson as "needs to be serviced every 300 yards".
The Yokohama Advan A046s had a 140 treadwear rating. High-output turbocharged engines (140HP/liter bone stock) tend to require frequent oil changes. High-output turbocharged engines also tend to run rich, basically using unspent fuel to help cool the turbo -- 14mpg is seriously what was observed under mostly rush-hour highway traffic. Premium fuel was required since it still had a near 12:1 compression ratio.
Those cars would never be offered via co-op, and I can't think of a car sharing service that even offers genuine bona-fide sports cars. They are rare, not mass market. They're incredibly noisy. Not very comfortable. They're "compacts" (in the US). They're only offered with manual transmissions (until recently). They'd be absolutely catastrophic loss leaders, especially with hooligans beating the piss out of them. They were engineered to be the most entertaining cars for the lowest transaction cost possible... but they very quickly compensate for their low sticker price with maintenance bills from a 911.
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What about those who purchase, say, a new Toyota V6 Camry, load it up to $30K, then put the minimum they'll allow down? Right there you're looking at a $550-$600/mo payment (and dealers won't care if they're getting paid and have the car off their floor plan). Add $100-150/mo in insurance. Assuming a 25mpg average, it's not hard to get right on the cusp of $1000/mo for the first 5 years, depending on how far people commute. That's with a vehicle that's around the average transaction price for a new car in this country.
My point remains that it's not hard to hit $1000/mo. I don't consider that prudent in the slightest, but a fair number of people experience just that.
As I said, I know how to not spend money on a car as well. I bought a Mazda MX-5 cash. Moved close enough to work to take mass transit. Negotiated secure garaging in my lease. Just use it as a toy. Between insurance ($120/mo), maintenance and fuel, $200/mo would be exceptional. I think I've spent $600 total on gasoline in the past 22 months, whereas 4 years ago, I remembered spending that every month on just fuel.
Rather than "trivially" making it to $1500/month, I think you pushed most parameters to the expensive side.
As a contrast, I have a 2001 Passat that I've owned for 6 years and do all the maintenance on myself. Over 6 years I'd estimate it's cost me $25k total, which ends up being $350/month. But then I've only driven 40k over those 6 years, too.