There are those who plan to keep their OM airplane, and
for those the resale value is irrelevant. There's a tradeoff between
resale value and OM as well: if you want to keep an old airplane
certified, be prepared to pay more and more money to replace
increasingly scarce parts, or to have them made and certified, or to
have them STC'd onto the airplane. None of that is cheap or quick. Over
the course of a few years the maintenance costs could easily outrun the
loss in resale value.
As an AME I have seen the wrong parts or even uncertified
parts on old certified airplanes. The aircraft isn't legally airworthy
in that case anyway and if an insurance company finds such stuff after
an accident they might refuse to pay out; what's the loss then?
And as an AME, if I owned an old airplane I would put it
on OM just to avoid the often ridiculous parts prices. All of us know
that some of those parts are the same (or close enough) as are found in
older cars, things like alternators or generators, voltage regulators,
belts, wheel bearings, engine instruments, seat belts, bulbs and so on.
Many others are easily fabricated; I spent years in a machine shop and
know just how easy it is to make some of those things. Homebuilders do
it all the time.
The big danger is the OM owner who has no mechanical
aptitude; the guy who puts hardware-store nuts and bolts in his
structure or repairs it with less-than-equivalent materials. He weakens
the airplane and puts himself and others at risk.

Dan