I was just reading this little bit of an interview with Nikon's Dirk Jasper, published on DPreview:
It mostly discusses the new D600 but I like the very last line, a point I don't remember Canon ever mentioning.
"Even second hand, or refurbished, a good lens is still worth its price, ten or twenty years later. Especially for enthusiasts, backwards and downwards compatibility is very important. Once you invest your money in a system it must be safe. You must get value for money. "
We do know that Canon glass, especially the L series stuff from the last 10 or 12 years, does hold its value extremely well. In fact, it's been my best performing investment ever!
But I take the context of Dirk's comment to imply the whole camera-lens system.
Even OUT of context, I'd like to hear Canon say, and mean, they intend to offer good value for money. Something I think the new 6D fails to deliver and something the 5D3 didn't exactly deliver at its intro price either.
If you had spent $20K on a set of Canon lenses in 1998, they would likely be worth well more than $15K today.
But, if you had spent $15K on similar Nikon AIs and "D" lenses in 1998, you would have lost most of it. Even though they may be good lenses, the old screw drive ("D") lenses, for example, have lost their value pretty quickly, and do not autofocus on Nikon's entry level cameras. Then, the first Nikon "g" lenses were optimized for APS-C bodies simply because Nikon did not have a FF body. This makes them less valuable.
Nikon is pulling your leg if they claim that buying their System lenses has been a good investment.
I really doubt that any Canon or Nikon bodies from 1998 have much value today, so its only lenses that have the potential to last.
Your comment about the 6D doesn't make much sense. It will likely retain more of its value than a Digital Rebel as years pass.