Replacing CDs isn't really relevant because we are discussing a scenario where the publisher goes out of business. The key, however, is to just take good care of your CDs/DVDs. At least with optical media the control is (mostly) mine. Are there sometimes manufacturing defects that will cause it to fail 5, 10 or 15 years from now? Yes. And the life of optical media is hardly the promised 100 years. But flaws are uncommon and there are other recourses should the media fail.
However with any form of digital distribution that requires contacting a server to use, there is, in my mind at least, a significantly higher risk of me being unable to replay a game 5, 10 or 15 years later. This is the primary reason I avoid digital distribution even though I agree its ubiquity is inevitable and it's all the more essential to niche genres that I tend to enjoy. Some very small publishers have designed systems where the protection expires after a certain number of years, which would be acceptable to me, but I don't know if this is a viable solution as I expect it can be abused.
If Steam were to promise (and perhaps they do...I don't know) that should conditions occur where people will no longer be able to verify their Steam products online (such as Steam going out of business) that all protections would be disabled, I'd make less of a fuss about this.
My secondary reason has to do with modding. I am passionate about modding games, whether the authors intended it or not. Systems like Steam limit the modability of games that aren't originally developed to work with Steam. There are significant problems with this with X3: Reunion, for example, even though that game was designed to be modded. Fortunately, I bought the DVD version.
Therefore, I boycott Steam as much as I am able, despite the fact that I don't know what a better solution will be. For as long as I'm able, I'll keep buying CDs and DVDs and support that form of distribution, flawed as it is.