I've been using the same keyboard for 10 years, and the only letter worn off is the N. And I'm sure it's because my fingernail on my index finger scratches it as I type.
Cut your nails?
And I always yell at DH for banging so hard on the keys - although I've never noticed any problems from it. It's just annoying!
But if you have callouses on your fingertips, you're basically "sanding down" the keys.
No other advice than that - except maybe the next time you purchase a keyboard, coat the keys with some kind of acrylic? Or get one of those keyboard covers like they use in auto repair shops? Or maybe, just maybe - you can learn to type without looking, and it won't matter if the keys are marked or not!
Good luck.