I'm afraid that very few of the people involved in the preservation and restoration of these structures and locations have any idea what they are really taking on.
Bill L at Geevor, and Tamarmole at Morwellham, have offered some very interesting and though-provoking opinions on the matter.
I suspect that like the railway preservation field, there are simply too many projects costing too much to maintain, for long-term viability. A preserved railway CAN be viable - look at the Festiniog, who seem to go from strength to strength, constructing complete new locomotives and other rolling stock, experimenting with oil and coal firing, rebuilding miles of line and generally running a serious railway with traffic densities unknown in any former life. In fact, you might say ( and some do ) that it is a misnomer to regard the Festiniog as 'preserved' at all, rather having been replaced by a reconstructed line along the main part of the original route and far beyond.
But, there are any number of preserved railways with yards full of deteriorating and unrestored - possibly unrestorable - locos, short running lengths ( sometimes little more than 'across the yard' or 'to the first level crossing' ) and generally, no real expectation of generating the funds for their plans.
A peculiarly narrow-gauge variant of this, is the 'reconstruction project' consisting of a short length of track and one or two of the ubiquitous small Hunslet or Barclay tank locos, plus a grandiose presentation showing how the original route remains largely open.
I suspect that the whole preservation sector will shrink considerably in the coming years, as structures deteriorate with time and the skills and funds to renew them are simply not available
''the stopes soared beyond the range of our caplamps' - David Bick...... How times change .... oh, I don't know, I've still got a lamp like that.