The biggest issue with the R10's age isn't the biocellulose drivers AFAIK, but rather the foam rings that surround the driver. I don't think there's much solid evidence to suggest biocellulose is any less durable than other driver material, though some people have assumed so given the nature of the material and how it's made. But even then we're talking about much longer than 20 years.
I see... Seems that TH900 should be very durable, since I gonna keep this headphone forever because it's the first well-made headphone in my collections, I like it much more than my HD700. It's a piece of art!