I don't know why to worry too much about long term longevity. I would use the best format that allows me complete control TODAY, and less damage to the RAW data, which is the original camera's RAW format or DNG. When Canon stops supporting CR2, if they do, it would be to go to a better format, and in that moment I would convert all my CR2 to CR3, or whatever they decide to call it. Same with DNG, if Adobe decides to stop using DNG and will provide, I don't know, QNF (Quantum Negative File, to put a crazy name on it), they will provide a DNG to QNF converter before RAW converters stop using DNG.
I believe that going to TIFF is not practical, since it doesn't have the ability to taking advantage of newer RAW processors, like noise control, demosaicing algorithms, etc., and it will consume tons of disk space.
When the moment arrives that I need to convert my files, I will also upgrade my disk drive system, and will convert each file and send it to its new location at the same time. Is not really advisable to simply save a bunch of files to a hard drive, and put it in a drawer. It might not even turn on at all the next time you plug it in. CD's, DvDs and BR are also not very durable media. I almost lost a complete CD collection because I though it was a good idea to put the disks in a folder, and out of their jewel cases. In a couple of years, the recording media started to break down. I salvaged most of them, but some were completely lost.
So, I think the best bet is to keep your data in disk arrays, and keep upgrading the drives as new technology advances.