The display is VFD, not LED. A VFD in a microwave has a hard life, because it is operating 24/7. Any VFD will dim over time for three main reasons:
- the filaments wear out, reducing the rate of emission of electrons, reducing brightness of the entire display, and sometimes resulting in blotchy looking display (usually brighter under each filament wire, and much dimmer between wires).
- the phosphors wear out, reducing brightness of the most-used digits/segments/characters.
- too-low voltage driving the filaments or the grids and anodes, due to (partial) failure of the power supply.
You can't do anything about the phosphors. You can increase the emission of the filaments by driving them at a higher current. This will restore some brightness to the display, but will also increase the rate of aging. It will eventually dim again, sooner rather than later. A power supply problem is best case scenario, because it isn't a failure or problem with the relatively irreplaceable VFD display itself.