I fully understand your plight about racking up miles too... I love riding vintage bikes. We have a VTR250 and an RD400, but spare parts are next to impossible to get regardless of how much money you have and every mile on them adds to the overall wear & tear.
I don't intend to veer off topic, but I recently had this type of thing put to me in such a way that it is changing how I approach "stuff".
(with respect a certain item you want to preserve) "If you died tomorrow, would anyone really give a rat's ass about this thing you hold so dear?"
The answer of course, for almost anything, is a solid NO. If you rode the heck out of an old bike until it was worn completely out, and then kicked off, would your wife curse you for ruining the resale value of the collectible motorcycle? Nope. Regardless of the condition, the bike would just be another thing to dispose of from your estate.
For me it is old motorcycle magazines. I have hundreds going back 20+ years. I couldn't bear to throw them away, and to me the $1 or $2 I would get for each didn't seem like nearly enough money, they were more valuable to me. I would rummage through them to read some article on the '96 CBR900RR or a Guzzi California that Cycle World/Pro Italia tricked out in 1991. I doubt even the author of the article or mechanics at PI remember the bike...but here I am digging through boxes trying to find it to read about the bike.
When the mindset above in bold was communicated to me, I had a sort of epiphany regarding these stupid magazines. They are just taking up space in the garage, nightstand, closet, etc. So far I have only given away all my MX mags from '85-95. It felt cleansing to do so. While I haven't disposed of any streetbike mags yet, I will either sort the good condition ones and sell them on EBay, or just scan in articles I like and toss them over the winter. It is wierd, realizing these stacks of colored paper and words mean only something to me makes it easier to not really care what happens to them. This philosophy is also starting to apply to my old 851, which has many parts that simply are not available today. I find myself thinking it would be more fun to see if I could get 100,000 miles out of it than it to be a perfectly preserved full-sized replica of a Tamiya model.
I think for me disposing of the old magazines is sort of like you riding the old bikes despite knowing that every mile makes them closer to a non-running parts bike on craigslist. If you think about it we are no different, every day is closer to our eventual tombstone.
So just ride what you want, as often as you want, for as long as you want. Fix it until it cannot be fixed, then find another one from a guy who considered himself a caretaker rather than enjoying its real intended purpose.