New vs. Old

I'm kind of curious and didn't want to hijack the other thread.

How big a difference does a new chassis really make in classes like FF and FC and why? If you spent $20k on a used but well sorted out RF97, what are you missing out on that would come with a new $50k piper? Is it super noticeable like a whole second better lap time or are we talking tenths here? Aero could be updated if new stuff is better, shocks as well (and often are, looking at a lot of the ads on here), there's a minimum weight so I can't see that being an issue. There's the zetec/fit vs. pinto/kent thing and yeah the new engines are pricey but assuming same engine and focusing purely on the chassis what else is there? Some big developments in suspension geometry or where the chassis is stiff and where it's flexy? Or are they just a lot more convenient to tune with a wider possible setup range? The difference is quite visible when you look at a chassis from the 80s or early 90s, but the late 90s/early 00s chassis all look pretty modern to my untrained eye. Just some pure technical curiosity, the same thing applies even to go karts as the newer frames are typically much better than old ones.