When any boat gets old and needs replacement of mechanical items, wiring, rigging it can be done. When a wooden boat needs planking replaced or a fiberglass boat needs a deck recore it can also be done. The bigger question is at what point does it become illogical economically.
But if the boat isn't unique, regardless of material, will cost more to rebuild than it will ever be worth when finished then it makes less sense.
Good points and close to what I was looking for when I posed the question.
To me (restorer of 2 glass sailboats - 26' and 43') the economics made lots of sense. I bought them for about $0.05 on the (new) $1, spent judiciously, consignment stores, Craigslist etc. and ended up owning boats for a small fraction of new costs. The Columbia 43 cost me about $45k (and thousands of hours of work) - what would a new Hunter 43 cost?
To me it becomes economically illogical when the boat is ordinary and the cost is approaching a new comparable boat. Following that logic I could easily have spent $150K had a basically new boat and still been waaay ahead of buying new.
And since when did economics play any part in owning a sailboat? They are ALWAYS worth less than you paid as soon as you take ownership. Actually, I think I made a few bucks (not counting my work) on the 26' racer after 9 years of ownership.
To further make my original point, any wood boats that had been as badly neglected as my two glass boats would have required MAJOR structural rebuilding - new backbones, ribs, planks, decking etc. They would have to have been built as new boats with some scraps of the old boats incorporated.
To connect this to my original point though - it seems to me that very many people would regard doing that hypothetical wood job as "doable" and even sensible but the glass boats I started with should have been scrapped - that's the attitude I fail to understand.