Older mirrors do damage very easily and it's surprising how quickly they deteriorate. I'm in the middle of restoring some pre-WWII SLRs and they will all have new mirrors by the time they are finished (two have already). (These are large SLR's 6x9 and Quarter Plate with big mirrors compared to modern cameras).
All SLR/TLR mirrors are front surface and originally used silver, this tarnishes and deteriorates badly cleaning accerlerates this. Modern mirrors use aluminium and this is coated, cleaning removes the silicon coating.
New mirrors are remarkably cheap £16 for my Quarter Plate reflexes and the same company can recoat any SLR mirror but you need a technician to remove it. I will get my Exacta mirrors recoated as they do cut the viewfinder brightness considerably.