The whole question is not so simple as converting to DNG. Yes DNG is openly documented, yes the file can be read, but that does not mean it can be correctly interpreted by other software in the future and the best you can then do is starting the PP of your images from scratch. Imagine a DNG with LR adjustments that in some years you open the file in another software, possibly non-Adobe. Even if the file can be read it will not look the same, as the Adobe specific developing instructions could not be strictly followed. Even if all software manufactures agreed on a common set of adjustments, let's say everybody had a Highlights slider and a Clarity slider and so on, which is unlikely, the results would still be different as the underlying algorithms would be different.
Disclaimer: I only shoot raw and firmly believe it is the best option, but one must be conscious it has some shortcomings related to the future of our images.