I think they dry rot just as much as typical latex rubber brake lever hoods do, so they just get more scarce and expensive as the years go by.......
Most used Kronos calipers that you see out there on bikes have some sort of dry rot issues, with their caliper covers. Only the NOS ones are usually the ones that look good.
If only they made them from silicone rubber based material, the would not be so hard to find in good condition.