I don't buy the argument that third cars just increases costs for teams. If there are fewer teams, they should each get a bigger slice of the revenues. The overall cost of populating a grid of 24 cars is also going to be considerably less if there are only eight different chassis designs rather than 12 because of the reduced amount of R&D, yet the overall revenues would be the same, so as long as the distribution of revenues isn't formulated in such a way as to kill off the teams at the back, it should be cheaper all round. What it wouldn't be is better.
We should remember that in 2008 a situation devloped whereby there wouldn't have been a viable grid in 2009 without the independent constructors. When Toyota pulled out the team just disappeared. When BMW pulled out it was only Peter Sauber's brave decision to delve deep into his own pockets that led to that team continuing. When Honda pulled out Brawn and Fry could have simply walked away, in which case that team would have disappeared as well. That would have left the Red Bull-Renault, Mclaren-Mercedes and Ferrari as the only manufacturer-backed entrants at that time. Under your proposed system of two cars each plus customers racing year-old machinery, we could have ended up with a 12-car grid - there were enough independent teams to give a bigger grid than that, but if they had been reliant on the manufacturers to supply them with a car, 12 would have been the limit.
I, for one, would not want the factories at Enstone, Silverstone, Grove and Hinwil downsized and de-graded to the point where there's no longer the facility to design and build an F1 car there. Once you start following that model the sport becomes entirely reliant on and dominated by the manufacturers, rather like DTM. That might seem okay when the economy is doing well, but next time there's an economic slump, they'll just all agree to not develop their cars for two or three years, and then they'll regulate in a development freeze and performance parity in chassis, just like they did with engines, to save money. You don't have to squint too hard at the automotive industry to see that cuthroat competition between themselves is something manufacturers will do almost anything to avoid. The top designers will go elsewhere. This is not what made F1 great.