[Perfectlap]

Well given the price of used air-cooled Carrera it wouldn't be a very good use of money if you're just trying to invest the money. Way better options for sure.

But a bubble is like pets.com shares, inflated wildly and at the end of the day it will never recoupe its highest price. That's not the case with the air-cooled Carreras. Their numbers are shrinking while the demand for them will always be strong. Particularly among bidders who learned to drive in the 70's and 80's. After the next economic downturn air-cooled Carreras will recoupe these high current prices and some will far exceed even those prices. I would say it's more a cyclical spike than text book bubble. But the long-term trend is obvious, Carreras will never lose you money if you own it long enough and you don't bid irrationally:
Paying $300K for a Magnus Walker 911 or a Singer really leaves little upside, it's priced for perfection.
At those prices their a long list of comps from Ferrari, McLaren, Lamborghini, etc.