One could think that a harvested, dead/dying mushroom loses its ability to glow, much like any glowing organic form loses when it dies in real life. Otherwise they didn't need any heavy lamps/torches in the first place! That is, if Luminous Russula illuminated well enough...
Additionally, dungeons shouldn't be so simple in their design that memorizing your way out would be any kind of easy feat.
Believe me, most gamers appreciate rather simple dungeons. Many could feel it's pain in the ass to escape one with many, many cells and layers. Also, how would the dungeons suddenly become even more complex? The longest of them are long enough already.
The map could be limited in dungeons though, making exiting these not well mapped areas a bit harder at least.