Two things... First, the burn in thing was overblown... Personally, I think what everyone was seeing was image retention... Which will look like burn in but really never gets any worse. Eventually, the same pixel sites showing image retention will begin to degrade, at which point it will start to worsen.
As for if dimming will work, it will. Each Pixel only had so much life in it. And that's a function of how long the pixel was active, and it's intensity. By dimming the buttons, you're reducing the latter.
Soo functionally, it will accomplish it's goal, but there's also a bit of a perception benefit here as users notice it, then assume it's working.