It's a rectangular box on wheels that rides on steel rails. What do you think they'd change so radically that they'd need to scrap them and start from scratch?
The truck assemblies get upgraded as materials technology and slightly better designs develop.
Exteriors can be repainted.
Interiors can be redesigned and refurbished, just like they do jumbo jets.
The other functional components of the cars have to adhere to the same standards that all other rail cars have to adhere to for interoperability reasons.
Therefore aside from them being totally destroyed in a wreck or rusting away to nothing for some reason or otherwise being totally destroyed, there's no reason to scrap them and start over.
The only conceivable reason I can come up with to scrap something like that is if the cost to rebuilt/refurbish it exceeds the replacement cost, and that's not a functional or operational reason, it's a *business* reason, something the beancounters come up with.
'Aesthetics' of the exterior don't matter. It's what the inside aesthetics are, where the passengers are, that matters, and those are only updated, again, for business reasons: if fewer people ride because they're not comfortable with the interior then they'll update the interior to increase ridership, therefore increasing revenues.
TLDR summary: This is a *business* and things are done for *business reasons*, not arbitrary 'aesthetics' reasons.