That happens occasionally to users, but on 3 lenses?? Something is happening, are you cleaning your lenses with something, or perhaps they are near a chemical. It sounds like there is a issue of something softening the rubber involved. They are older lenses, so there may have been something at low concentration working on them for years. Don't apply any chemicals to the lens that disolves the rubber, it may get inside the lens and dissolve other plastics like wire insulation. I hope not. Tale the ring off and clean it away from the lens, If I were not replacing the ring, I'd consider wrapping it with self adhering silicone tape. No adhesive is involved. I've used it when the 12 or so rubber grips on my Benro tripod all suddenly fell off and they claimed that there were no replacements. It does not provide the same grip, but it does work and has a wide temperature range.
It is used to cover electrical splices on fueled aircraft where heat shrink is too dangerous due to a potential fire.
https://www.amazon.com/X-Treme-Tape...lf+fusing+silicone+tape&qid=1548181031&sr=8-3
That is neat tape! I'd been given some before but without a label and didn't know what to order to get more, so yay! Regular electrical tape turns to goo but this stuff is wonderful.
But for this sticky coating problem, I do know a bit about what goes on.
First to allay your concerns: These lenses have always been stored in air conditioning, and have never needed cleaning by anything but eclipse fluid on the lens elements. The 28mm and 70-300mm have been used in very different condition, and with the 70-300mm seen little use since I got a 70-200mm f2.8. Now that I think about it, they've all been on hot Orlando vacations but weren't left in hot cars. (heat probably is a factor in the time to goo)
Taking the ring out to clean it isn't a bad idea, so point taken. I decided to chance it with the 28-135 both because it's obsolete and not too valuable anymore and because I've used this cleaner on plastic extensively without trouble. I use it at my day job to clean dirty computer electronic equipment, and have only seen it be dangerous to painted labels. But - that's not a lens with lens coatings etc. If the 70-300m came apart as easily as the 28mm I might have taken it apart to be sure, but slow work with q-tips worked well and the other lens cleaning went fine. Taking the ring out would be safer, but there is some risk is disassembly too. (for those following along - remember you must have JIS not phillips screw drivers, and tweezers for the flat flex)
The consistent thing about the lenses is age. The coating on the hard plastic focus ring failed, the rubber band style zoom rings are all fine. Both would have been exposed to the same conditions. The replacement from Canon isn't coated, which made me wonder if they'd stopped using it. I checked though, and the brand new 35mm IS has a coated focus ring so that's not the case.
Outside of this, I've had synthetic rubber fail this way before. Foot pedals on a '92 Taurus SHO, and almost all the interior panels of my '01 Benz SLK was coated in this material and it looked like the car had leprosy after about 5 years. Actually I've got a 1st gen Razer mouse here that did the same thing, and was my test case for the Krud Cutter cleaner. The same thing has happened with other mice that had this kind of coating. (it's nice at first though)
A PHD polymer chemist explained a little about it. (simplifying for me) He said that in order to get the suppleness, the material isn't really chemically bonded internally. It's flying in formation instead. To make the trick work the purity, exact amounts and precise temperature are all crucial. It stays sticky goo if they're wrong, and turns back to goo if the conditions were close. Lasting for ~15 years would indicate it was almost right, and you'd expect different batches to vary so that'd explain one reason why it's not the same for everyone. I bet the Canon engineers know exactly what's happening and why.
Which is a long winded way to say I think the problem is that the material used in the coating has longevity problems in practice.