I believe that RVSM will be the final nail in the coffin of some of the older aircraft. The Lear 20s, Sabre 40/60, Jet Commander, Hansa, Haker 1 - 400, etc. While there are companies working on making the aircraft RVSM, there are many other items that will park the aircraft.
When the price gets too high, they will be parked. Take the Lear 20 for an example:
RVSM, programed to cost 150K. It is not yet certified. So for right now it is a crap shoot. Price? Until they get the first half dozen or so out the door, that price is fluid. And it depends on what has to be changed. Could include A/P (100K +) etc.
Engines, most experienced operators plan on an overhaul cost of 400K per engine and then they keep their fingers crossed.
Inspections, the 12 year inspection is minimum of 100K. The 12,000 hour inspection averages 200K. Langing gear inspection (12,000 landing) can be as high as 70K.
Add in the difficulty in getting parts. Insurance costs and so on, most owners will just park the older aircraft. Run them out as best they can and move on.
While a few might bring these aircraft into RVSM standards, I don't believe you will see a lot of them.