I have to go back to many experiments we conducted years ago. Why petroleum products where the best way to reduce the rate of the existing oxidation of steel. If you put a dob of grease on a sheet of metal we found that over time the oil component or lower viscosity component slowly flowed away or migrated from the grease. I cannot remember the actual radius. It is enough though that the same is going to occur in the rear cv joints and it will reach the boot.

So you are right if the boot is rubber and that I doubt because of the stiffness of the material. Total rotting of a replacemnt aftermarket boot has not been reported on site either. The makers would not make them of straight rubber and expect to remain in business either in my opinion. I would certainly contact and question the manufacturer of them if rubber is suspect.

I am unaware if the split boots have been redesigned to compensate for the uneven flexability that has semi doomed them from the start. Taking the time to remove and refresh the lubricant in the existing good joints on a car is time well spent. A few replacement clamps should do it if your boots are still good. Many of those joints have not seen any attention for at least twenty years.