We're talking about slightly different things here, Bryan. I was specifically addressing the point about saving everything as TIF if one wanted to salvage all your adjustment work, and for this purpose I don't have any worries. The DNG can store an updated JPEG reflecting your adjustments, and plenty of low level programs grab those embedded JPEGs and print or output an image that's effectively-indistinguishable from one produced by Lightroom.
Your real query is about a risk of DNGs not being readable by any program. First I think we need to confine discussion to mainstream programs for humans - not the butt-ugly utilities out there that will indeed read the files but aren't for most people. Second, we can't worry too much about temporary bumps such as how C1 didn't read DNGs with fast load data, because whenever Adobe improve the spec there will be a lag before others read it properly (that said, apps don't need to reject the entire file simply because it contained supplementary elements they don't understand).
Even if we're assuming Adobe are beyond the pale, for whatever reason, I think DNG will always be readable in mainstream programs, just like CR2 or NEF will always be readable in another program (that's not why DNG is so good!). But the doubts are more about the choice of programs and whether you can still use the one you want to use. DNGs will always be readable in some program or other, but are they readable in the mainstream program you prefer to use, and secondly how well are they read? Raw files give you a few more options, but I'd argue you'll still have enough options with DNGs.
Right now for example Aperture handles DNGs exactly if they were proprietary raw files from the original camera, but do you want to switch OS? OK, that's not such a big deal nowadays. Or take how C1 handles DNGs as second class citizens, by which I mean if you import a CR2 raw file, C1 recognizes it's from a specific camera and applies all sorts of C1 magic, but if it's a DNG from the same camera they simply treat is as a generic DNG. It doesn't look too bad, but you're not getting the best that C1 offers - for instance, you can't apply their lens corrections. I feel they are bone-headed over DNG and all my contacts with them over the years indicate it's a firm belief coming from those who drive the product, but I'd hope their recent attempts to sell C1 to Lightroom customers (apart from importing LR catalogue, so much of their marketing and presentations are geared to trying to gain LR users) are signs that they will have to change if they want those efforts to succeed. That's two mainstream apps, and I'm rather expecting Google to enter the market and handle raw data, and I can't see why they wouldn't handle DNG for the same competitive transition reasons.
So I'm optimistic, but I've always been in the camp of advocating DNG and keeping raw files as an additional backup.
John