The amount of dust that would be necessary to incapacitate a mechanical keyboard would probably coincide with the amount of dust necessary to fill all the space of the key travel.

Most mechanical keyboards are basically protected from dust entering the physical area of the switches due to a hat-like dome formed by the keycap itself. On top of that a lot of the switches are closed and rated for minor dust exposure (cherry's can with stand some water and dust internally)

I've never heard of a keyboard failing because of dust with the exception of Apple's MBP keyboards.