An actual button failure would be a defect. They should last as long as the rest of the
device, given normal use. Now if you use it in some way that subjects a key to some
abuse, it is only plastic.
If you take the device apart, be sure to keep track of where each screw goes. Some are
a little longer than others, and there is one that can effect the use of the "0" key, if the
wrong screw is used.
I seldom use the slider, I prefer the shiny page turn button.