I've been doing classic cars for well over 20 years and even in the classic world there's a strong debate over how to define a classic car. Whilst almost everyone agrees that my mini and stag fit into the description of classics they have very little in common, the mini was build in vast numbers and there are still thousands on the road so rarity can't be a deciding factor. The stag had poor build, poor reliability, and not much desirability when it was current but good looks and a strong following amongst a small number of enthusiasts ensured its status.
When I started the e-type and Heeley owner types poured scorn on the XR and GTi cars at shows, now they are seen regularly at most shows up and down the country. People used to say you couldn't fiddle with fuel injection, abs and ecus but we do. I don't think it will too long before obd2 will become hackable and we can fiddle with our moderns with laptops and iffy software. Most modern cars I've worked on are easy to spanner compared to old stuff so I don't think general maintenance and replacement will be an issue. Bumpers, panels are all easy to replace and welding is welding (Ali excepted)
Will the z be a classic in 20 years?, well folk said the mx5 would never attain classic status 20 years ago, so I recon it has a good chance, along with the TT and SLK. The Boxster is already there!
Cheers