Due to pace of change playing older games can always be tricky - digital's got nothing to do with it really.
As for new generation it would of course depend upon the HW but there's no reason for Sony/MS and anyone else (Steam for example) not to hold a record of your past purchase and (assuming its compatible) allow you to download and play it on a new console.
Given the current importance of Live/PSN and the success of Steam I doubt digital games (particularly popular ones) will just get 'turned off'.
As for selling on digital games... it would be nice, but I can't help thinking one of the big attractions to publishers of DD is preventing you from doing this. I really doubt we're going to see it as an option, not unless they get some money - so for example you re-sell your rights through them to a friend at a discount: they sure aren't going to back anything for free given how much they hate the fact the don't see a single dime on every second hand game currently.
Try to be reasonable... its easier than you think...
