Actually, I'm almost willing to put money on the fact that you'll be wrong. At least in some cases.
I think that digital downloads aren't going to be able to get away with being stuck to one piece of hardware like physical games have; people have far too big of a difference in perspective when it comes to them. Your digital music can move to new iPods, your digital photos can move from computer to computer, digital content is seen as something that flows from one device to the next because the obvious concerns of how a physical piece of content is going to interact with newer hardware doesn't exist. Even if it isn't an educated opinion, I'd be willing to say that that's how a lot of people see it.
The biggest ray of hope I see in this is... urg, Microsoft. (So much as I hate admitting that.) I don't know all of the technical aspects, but from what I understand, Microsoft is using some level/type of Direct X on the Xbox 360, meaning that game engine stuff is going through Direct X instead of straight to the metal. That's a very, very good thing, because it means that games that are programmed right don't have to be console-specific. New flavor of Direct X on new hardware that can run and interpret the old code, and voila.
Simplified version of what's going on, but I've got a lot of hope for stuff like Xbox Live Arcade games in that regard. As much as I initially hated the idea of consoles becoming more PC-like, the truth is it'll have huge benefits in the long run. Problems could still crop up depending on what kinds of things are hard-coded into the games, but it seems like with the 360 Microsoft was smart and got a lot of that stuff out and into the hands of the OS directly. For example, Halo 2 doing friends lists the old built-in way is the reason why we still can only have 100 friends, so problems like that occurring could hurt the length of hold long older XBLA titles are supported. If the games can be run in a sandbox, or are independent enough from the OS and its features, that won't be as big of an issue.
Sony and Nintendo are another story. If we get Wii HD, well, things are safe for a while. Of course, they're not at all, because Nintendo currently offers no way for you to move your content from one piece of hardware to another, because they have no fucking clue what they're doing digitally. Sony is a wildcard, because unlike Microsoft they seems to put no real emphasis on producing consoles that are a mix of hardware and software that follow some overall strategy. For Sony, it's more a desire to get out some crazy "uber-leet" system and force people to learn things all over again. I have not one clue what's going to happen in the transition to their next system, especially now that Cell development is looking a little shaky.
It's a very interesting topic. Do you buy retail games for a console with the expectation that they'll be playing on the next batch of hardware? I don't think most people really do. And yet, we concern ourselves about that far more with digital purchases (which, funny enough, cost far less). This obviously will become a bigger question once gaming goes more digital, so for now it still feels very much like an experiment.
For me, I know that at first I didn't worry so much about this, but now I do more (in terms of the current generation of digital downloads). The inability to even move games to a new Wii has almost completely stopped me from buying WiiWare games, and the same with DSiWare (unless they are things I really, really want). I still buy PSN stuff, but the more I think about it, even if I have no solid proof as of now, I think I feel more confident buying a game on XBLA over PSN.
The thing is, though, I think with digital downloads, developers and publishers are going to have far more reason to want their games to stick around. If you take something like the move to Xbox 360 from Xbox, they have reason to not want you buying Xbox games, because then you aren't buying their 360 games. With digital downloads, there's no shelf space to fight over, so a game like Pac-Man CE could theoretically be a viable sales product for the next 10 or more years. Remaking the game for the Xbox 720 and then selling it for the same price makes little sense. You do of course get to the question of saturation, and wanting/needing people to re-buy your product again and again on different formats. If you're somebody like Lucas, you want people to keep buying the same Star Wars movies over and over again on different formats instead of buying once digitally and being done.
For gaming, though, I think that there's still a lot to be gained from having one copy of the game you can continue to sell for years and years without having to tweak it for a variety of different platforms.

