Hi,
One solution would be to put clear nail polish over the keys of a new keyboard.
There are also stickers for the visually impaired that my stick to the keyboard better than the ones you tried. Look here
http://www.hooleon.com/prod-vis.htm
I'm not sure why your letters are wearing off. In my 20+ years working with computers, I've only seen this 3 to 4 times.
Right now I'm using a 3 year old laptop that I work on for about 5-8 hours a day, 7 days a week, and the letters are as clear as the day I bought it. Maybe it's my baby-soft hands.
Have Fun,
Jeff