That's my point. The market for an old TDI, or MB diesel, etc. attracts a very specific person. Highly skilled/experienced mechanically, but understands that mechanical things should be used, and they use them, repair to better than new, and upgrade as needed. Fairly significant sums of money can be invested, but knowing that doing so is cheaper than buying new, and the comfort of knowing what was done was done correctly.
It's a very different thing from the restoration "drive on sunny Sundays" crowd. The cars we turn our attention to are cheap to buy, cheap to fix, and easily replaceable. If I want a rust free car, it's a simple matter to jump on. SouthWest flight and go get one. Or ship it in.
There's not that many of us, compared to the supply of good used vehicles that are also in relatively low demand. It's a good time to be TDI people. 10yrs ago was a great time to be Mercedes 123 enthusiasts. They have largely dried up, supply-wise. I give MKIV TDIs another 5-10 years before parts and decent used cars become difficult to find.