Am trying to keep my 1995 saturn wagon forever. (211k now) I have spare body panels I robbed off a spare chassis I had crushed. A liftgate alone is $600 new and impossible to find used-- I know, I was rear ended five years ago.
I have a spare engine/transmission in my garage. I swapped the engine cradle/subframe with one from a 1999 last fall when corrosion got the original part. Rust seems to be eating only this-- the unibody floor pans survive very well-- and I plan to visit some southern junkyard to get several more K-frames sometime. The platform went through few changes in an eleven year run so mix-breed mongrels are easy to assemble if one doesn't care about originality.
Several years back when gas was shooting up I figured it got better MPG than newer stuff hobbled by NOx emissions, so I re-ringed the motor and did new struts. I don't see too many 2000-2010 vehicles that excite me technologically. In a way I feel like a previous generation must have felt in the 1970s when compression was dropping, cats strangling exhausts, net HP just came out, etc.
I don't have to undergo any emissions inspection, it is only for OBD-II cars here. Just slithered by. I can drop a 7-years-newer engine/transmission in if I wanted.
My approach is not to have it last forever, but to last until gas is around $7/gal, (I figure in the next ten years) by which time someone will finally snap and let euro-spec small diesel cars like the VW Polo in the States.
My motorcycle, a 1982 Virago, never sees winter. At 11k miles (2.5k/summer for me, not the PO) it has a lot of life left. It's relatively raggedy and undesirable to others, so worth very little, and can hide on the far wall of my garage not taking space. It may last a very long time.