Any rubber is going to rot if it's stored poorly. One bit of rubber that always is known to melt is the rubber feet on the charger for the PowerBook 500 series laptops. It ALWAYS melts. I've got one that hasn't done so yet though. Why? Because I believe that it was stored rubber-feet-up for however long it was in storage in the past. I left it for around a week, feet down, and it got slightly stuck to my desk. It's pretty clear to me that if I left it for a couple months like that, it would melt like all the others have. My assumption would be that it is chemically fragile enough that contact with surfaces will cause it to rapidly chemically decay.
Case in point, my PS/Note 425 (sold as the ThinkPad 350 as well) has the feet melted on one side but not the other. It was stored in a box sideways, with multiple other laptops in a garage for probably over 20 years. During which time, the front side feet were contacting something else in the box, while the upper ones weren't. That just about proves my theory for me.
This does get more tricky when you consider something like a 700-series ThinkPad that's completely coated in the stuff though. Environmental aspects clearly also play a role, which explains why you can still find non-rotted 701c laptops if you look hard enough. Don't you have a couple solidpro? I think I saw your post on the ThinkPad subreddit a while back. I assume you're the same solidpro anyway.
My rubber-coated T series laptops are all still ok, although my T30 has some strange discoloration on it. I can clean it off but it always comes back later on.
