hi Mark,
I have no experience with piano (hammer-like) keyboards, but I think there's a big difference in the way how keys wear out.
As I understand your explanation, keybed on piano keyboards can wear out (become louder) over the time, if player hit keys harder than "normal" -consequences you mention (felt & other mechanical wearout ) make sense to me.
Let me explain what I saw when I took my SX700 apart...
The difference is, PSR-SX keyboards have extremely simple key construction and it's impossible for them to become louder over the time, because player would smash keys too hard. At the bottom, keys always hit on rubber pad (also serves as contact) which never wears out, and in the top position (not pressed), the key is hanging in the air. That is, there's never a "thump" or "clack", no matter how fast/hard the key is hit vertically.
The problem (at least that's true for SX-series) is horizontal key rattling. It's to be expected that there are some tolerances in horizontal key movement. To limit horizontal movement (so key doesn't rub the key next to it), there's a notch on each key (where key "travels"). Obviously, each time when key is not hit perfectly vertically (practically all the time), the notch will be touched -which manifests as audible "clack".
To prevent that noise, Yamaha decided to put a small amount of grease into that notch, which serves as a damper for horizontal movement. And now you guess it: as we play, the grease gets moved/pushed out of notch over the time. In short: player has no influence to prevent that to happen.
Yamaha soon realized that problem (too late, though) and published intern service bulletin where repair process is described and told which special grease must be used. Because if grease is too thick, then keys will become sticky and if too thin (liquid), then there won't be enough dampening -obviously solution can't be (and is not) long lasting.
Bogdan