Strictly speaking, being open source is not enough, by itself, to guarantee the long-term viability of DNG files because you still need a program and an operating system to run that program on to get at and do something with your DNG negative. But that's probably semantics. At the moment, that seems like a pretty safe bet. Nothing in computers ever seems to go completely away. It would add a minor complication if/when an upgrade to DNG is released. There will probably be software to process DNG for a long time to come, so long as there is a need.
But C, N and others could go a long way toward negating the effect of DNG (if they wanted to) by adopting a strategy of releasing their proprietary raw formats into the public domain after, say, 5 years of the release of a camera model. That gives them time to make money from unique proprietary technologies and (maybe) software sales but still addresses the long-term worries of creators of the images. So far, there's no sign of them doing that although since their raw formats have been deciphered anyway, it may be an exercise that they see no point engaging in. This is idle speculation, of course, I have no idea what the manufacturers' strategies are.
I detect some vehemence against the manufacturers for acting in a proprietary way. I understand this completely. Our images do belong to us and we don't want to be held to ransom to get at them. But it's not 100% black and white, I don't think. In the film days, everyone was better off by standardizing on common colour development protocols. But nowadays, when each one of us can build our own proprietary darkroom, the manufacturers could take the attitude that preserving our images for posterity is our problem, not theirs.
If you write a novel using Microsoft Word, it's good practice to save an ASCII text version somewhere. There's a better chance of being able to read that text file in 50 years that today's Word version, I would guess.
If a manufacturer gains a marketplace advantage over competitors by bucking that trend and fully supporting an open format like DNG, that might change their collective minds. So long as it doesn't affect sales, though, they may decide that they don't need to do anything.