Well of course if you never moved it, it would never break. The fault occurs from the wiring on the shifter. At the bottom of the shaft, there is a small block, which has the wires for 1st,2nd,3rd,4th,5th,6th and reverse soldered into it.
These wires then go from the shaft, and up by the d-pad, where they are fixed into a board BUT this can be unclipped easily. This same board also house's the buttons. The reason the gears go is because the soldering is very poor and brittle, and can easily break apart.
When this happens, then the wire obviously lose's contact which is why you lose gears. It has been heard for the contact at the board to break (happened on my second one after taking it apart to investigate) but i believe this is a rare fault. If that does happen, then its beyond repair as the contact breaks away from the board.
If its just the wires at the shaft that have gone, then this is an easy fix. You just take the gater off, and re-solder the wires back on. Obviously with a warranty, this would not be needed.
Sliding the shifter up and down the desk would do nothing to harm or help the lifespan. The wire is flushed into the casing, so is therefore level with the desk and makes minimal contact.