@panos
I think you already know the answer to your simple question, but for confirmations sake: no, keyboard owner can't influence the failure of electronic parts. But that fact changes nothing. After warranty period, we are on our own.
Even if the motherboard failure is not something that happens often to psr S series
how much is often?.. 1 out of 100, 3 out of 1000? The thing is, not every keyboard owner reports such things on forums. We don't know actual numbers, so we tend to think "it's something that just can't happen".
It's easier to get a picture with PC components (mainboards, graphic cards, routers, etc.), because there are much, much more people using it and report their experiences. Over the time, certain brands become "flagged" as unreliable, while other are recommended by most owners. Of course, "recommended" brand can fail too, however there all less chances for that to happen.
Speaking for me, if certain (expensive) product stops working normally shortly after exceeding warranty, I never decide for the same brand again. Thus, using it only at home, I give my PSR to work flawless for at least 8-10 years.
Just sharing my thoughts