No such thing as an investment if you plan on using it, every time its off the shelf its subject to wear and chance of damage.
true, its unlikely that todays dslr plastic blobs will be worth anything down the road, we've already seen what happens with AF 35mm slrs. A beat to death manual camera will bring more than a 90s AF slr.
The saving grace with digital is that it medium is not in danger of being discontinued anytime soon and therefore destroying any value it has.
Proper batteries for Nikon Fs etc is why the prices on them is staying low and F2's are staying up there, stop film production and all film cameras value will go through the floor overnight.