I've been thinking about potential oil tank leakage as a Konnarock maintenance problem. Given that the near term risk of an oil spill is low, but that in the long term the risk of catastrophic clean-up costs is real, the decision that we make implicitly or explicity how much longer we should postpone taking action.  I see the following options:

a) Es auf uns zukommen zu lassen. That is what my parents would have done. Die haben 1933 die Nazis auf sie zukommenlassen, und 1951, C. Ross Ritchie and the Board of American Missions. I then spent 10 years, and in a sense, 50 years of my adult life, protecting them from the consequences of their capitulation to fate.

b) We could presume to rely on early detection. In Belmont this is very satisfactory because we have video access to the bottom of the oil tank.  In Konnarock we could potentially rely on video surveillance: 
b1.1) of the west wall of the shop, 
b1.2) of the floor of the furnace room, and/or 
b1.3) the leaky floor of the furnace room, to give us "early" warning of an oil leak.

Hypothetically electronic measurement of the height of the surface of the oil could be monitored to give notice of a leak.

b2.1 although there is an "app" sensitive at best to 0.5% for ultrasound monitoring of above surface tanks, for underground tanks no such "app" is available, and even if it were, the sensitivity is veryt likely to be inadequate.  0.5% of 2000 is 10 gallons soaked into the soil. So what would one do?  It would be too late to lock the barn: the horse would long since be out of the door.

b2.2 I also entertained the possibility of improvising a homemade manometer attached to the intake pipe of the oilburner, but concluded that the surface tension of the oil, if nothing else, would make the readings unreliable.

With all techniques of "early warning" the risk of false positive or false negative readings would would be unacceptably high.  What would we do then? Even if we immediately emptied the oil tank, we would still be required to remove the then empty underground tank, and remove any contaminated earth, processes that, as you noted might cost more than the value of the house.

c. The most feasible solution, I concluded, would be to empty the underground tank.
c1 - by running the oil burner until the tank was dry, or
c2 - by having the oil tank pumped dry and paying to dispose of the oil, or
c3 - by transferring the oil in the underground tank into a set of five 275 gallon tanks arrayed side by side at the eastern wall of the garage; and this, indeed, is what I consider to be the most promising scheme.
 
In order that the house can someday  be sold, tt requires to have a heating system. Natural gas is unavailable. Propane is inordinately expensive.