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.
goes both ways, if you bought in to a bunch of EF-S glass and now decide you want to go FF, start all over.
OTOH, even if you have crop spec (DX) Nikon glass you can still use it on a FF camera in crop mode.
As for their older AI/AIs lenses, funny you should mention. I just loaded up on a pile of primes for dirt cheap in the used market and many of those lenses perform fabulously even on the D800 and even the crop body D5100 - for what *I* need them for.
Some of the old screw-drive D series Nikon lenses still focus faster than many of today's AFS lenses when on an appropriate body.
I avoided buying into Nikon because of their clunky looking and confusing lens history. But a soon as I got familiar with it I found it's not complicated and there's a LOT of good old glass out there that STILL WORKS on digital whereas most of my old canon FD lenses, with a converter to EOS, could not perform at all on a digital body. Inadequate coatings caused massive flare/coma rendered them useless. Converter partly to blame but frankly i can also use old Nikon glass on my Canon EOS with a much simpler adapter and get superior results too.
back to my original premise.
The 5D3's a great camera but i think it was a bit overpriced at launch considering the meager IQ improvements at low ISO. It's now coming down to a more commensurate level. If I didn't have a 5D2 already, i'd likely buy one.
6D, in comparison to the D600, doesn't offer as much for the same price.
6D will likely drop in price until it reaches an appropriate "value" while the D600 likely won't drop in price since it's providing value and performance where it is.
tougher to compare 5D3 and D800, rather different cameras at similar prices. For me the D800 was a no-brainer value decision, it fulfills needs Canon can't and does so at a palatable price... Which is likely to remain the same.
Put another way, Canon's stuff is overpriced at launch.
Pent up demand after long delays likely leads to plenty of sales tho so they're still ahead that way.