With the right equipment, which is usually beyond the means of Joe Average, mold & fungus can be removed safely, but it is economically not feasable a lot of the time. Unfortunately mass production has lowered the price per unit so much that individual labor time has become prohibitive. Its actually cheaper to just buy a new one than fix the old one, unless it is in some way collectable.
Element seperation is a different story. The old dying Balsam has to be chemically dissolved (takes days or weeks to happen), the glass repolished (very risky, no-one will guarantee this type of work), & then recemented with fresh Canada Balsam (again weeks of curing time) & then reassembled & tested on an optical bench. Costs for this are astronomical.
Allan Quartermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”