This may be entirely irrelevant to Topre, and whatever sort of rubber substitute they use for their domes (silicone?), but I know that auto detailers are convinced that plastics and rubbers dry out over time as the oils within work themselves out, evaporate, etc, over time. There's no arguing with the results either when you look at those gritty old looking bumpers and somebody hits it with a hair dryer or heat gun and lets it soak up some good old ATF. It seems to restore its original color and some of its flexibility, and it lasts afterwards. I have tried similar things myself with rubbers and plastics, and it has worked wonders on 50+-year-old rubber recoil pads, grips, etc, that felt as hard as rocks and were beginning to crack.
With that in mind, if the domes similarly stiffen and/or become brittle with age, I can't imagine a decent synthetic lubricant could hurt matters. ATF is used by a lot of people, in a lot of businesses, for its combination of an extremely low price, mechanical lubrication properties (primarily between metals), and the fact that, being a synthetic oil, it doesn't tend to break down/gum up over time. Not that it is in any way suitable for keyboards.
