The format longevity question is impossible to answer conclusively. But it reminds me a bit of the best advice I've heard for preserving ones' personal artwork: become famous. Famous artists have their work in galleries and museums, and a whole industry exists to support them. Similarly, I imagine that the most popular cameras will have the best chance of ongoing raw converter support, even if it's only as a DNG-converter style program, while others may be lost to the sands of time.

And every camera model has a unique raw format, even if the file extension is common to the entire brand. Software that works with the NEX-5R won't automatically support the NEX-5T, Canon 70D needs to be supported separately from the 60D, and so on. DNG files are much more universal, although each converter will give different results.

One other advantage of DNG worth mentioning: there's only one file per photo. Proprietary raw files can't be modified, so programs like Camera Raw need to create additional files just for keeping track of the changes to be made to the file when it's exported into a pixel-image. These sidecar files that contain all of the editing instructions are unique to each program, and if they're lost then so are the changes. With DNG these editing instructions can be written directly into the DNG file, increasing their portability and making backups and archives more manageable.

But wait, there's more! One reason NOT to use DNG files is that some programs can't work with them. DxO Optics is my preferred raw converter, but it can only do its magic on original raw files, not converted DNG files. That's critical for me today, and who knows what might come along in the future.

So, to make a short story long: convert to DNG on import for your working copy of the file, but also buy a basic external hard drive and check off Lightroom's option to archive the original raw file on it. When that drive's full put it somewhere safe and start another one.