I have seen 10-12 year old P100's on a network that have been used without mercy for their entire existance, by people who smoke (ie nicotine stains on the keys) and eat and drink at their work stations. All the keys were still there. Totally gross looking, but still there.
Honestly, in all my years around electrical stuff, including the dozen or so pocket calculators I have owned, I have never seen the keys wipe off clean. Even my TV and stereo remotes are in pristine condition - no, I am not one of those people who wrap their remotes in shrink wrap to keep them clean.
Don't worry, type happy...