I dont believe the chassis wear out as much as the bearings wear out. you get a new chassis, you get new bearings...
One year we had an older shifter kart with the front loop worn thru 90% and 8mm kingpins and got on a new one with 10mm kingpins, and was reported they both drove the same, and both ran the same speed. That said, another chassis brand did turn to crap after a few races. Basically, if you cant adjust the kart and have it respond then its time to change.
Al, your correct with the European way. Some of those guys would change mid event, and some events let us check in 2 chassis, mostly a "rain chassis" so that if it rained you had one already set up and didnt have to change the dry setup.
There are problem with only going to the cheap copies of karts, and or only buying used. One, the new innovations would stop. The sponsorship dollars would dry up and info would stop flowing. The competition between manufactures actually do keep some prices down. Think about it, IF all there was was Phantom, how much would they cost? IF the only reason to buy a Prowler was it was cheap, why buy it if you couldnt win with it? So, the prowler guys put in work to beat the Phantoms, and the others included too.
But put it all into perspective, How much does a Late Model cost compared to a NEW kart? I once stopped by the racecar shop and they asked why not run cars, it cost the same per event as running the karts. (which was true at the time) but running the local late model class i couldnt go to any other tracks as they all had different rules. Plus, they couldnt compete with the argument that if i wrote the ENTIRE machine off id only be out 7K or so. Cant build a car for that much.