I think each team should have 2 cars and just alternate them from round to round. Just my .02
Every car, even if they are an identical pipe layout, is different. They have different response characteristics, they flex a little differently, and they have slightly different weight differences. Crew chiefs hate bringing out a new chassis, it's a last resort. And drivers get comfortable with one car as well.
Also, there are more than a few teams that can barely put together one car (viz Haddock, Caramesines, etc.), and many can just afford the main car and a backup (viz Wilkerson, Zizzo, etc.). You're asking them all to have
two and a backup if they need to go more than two rounds?
And what about the teams? Maybe in the morning before round 1 you could prep two cars. But let's say you win in round 1 in car A (with team A). You scurry back to the pits, hop into car B with team B rush up for round 2. Back in the pits, team A is rebuilding car A just in case you win. Where do all those people come from? Who pays for them? And what business would keep a team of people around
just in case you go to the semis (a 25% chance)? The teams that can't afford all these folks are at a very real disadvantage, no?
Then where would they put them? Pits are crowded as it is, I can't imagine what it would be if every pro pit was twice as wide as it is today...
This is really not a workable idea...