The only constant in life is change, so planning for that change is very important. I have some negatives I inherited from my grandparents that are over 60 years old (circa 1948-1949). These negatives are on a non-standard film (by today's standards), larger than 35mm but not quite medium format--127 format. My scanner can still read the format, although I have to be careful handling it.
You aren't going to find a single digital standard that is going to outlast the film, but there are still things you can do to hedge your bets.
- Store your image files in a vendor neutral, no-loss storage format. TIFF files with 16 bits per channel (48 bits per pixel) will keep all the detail your camera can dish out. Most sensors don't do more than 12 or 14 bits per channel in RAW. Just be careful not to use JPEG compression in your TIFF files.
- Update your stored files over time. Sometimes a change to the spec has to be made that is not compatible with older versions to fix new problems. By batch upgrading your digital files every 5 years, or every decade, you can stay current enough.
- Be a ruthless editor. Before you update your stored files, take the opportunity to weed out the pictures that just aren't important enough to keep. Your daughter's birthday party may be an important event, but you don't need every single picture from it.
The pictures you have that survive 10, 20, 30, 40, 50, 60+ years should be the ones with the most importance. They can be historical, sentimental, or just really good timeless pictures. Some pictures only have a very limited time of significance. For example, I do sports photography at my alma mater high school, and the pictures I take only hold significance until the children in it graduate. That's when I bundle them up, and give them to the kid's family. (it's a small school).
Another option to avoid digital obsolescence is to avoid digital altogether. There are some services out there to convert your digital pictures to film. Depending on the emulsion, film can resolve up to 2400 lpi (line pairs per inch), or roughly the same as 4800 dpi (dots per inch). It's a big expense, so you should only consider it for the pictures you really want to keep long term. It also takes up a lot of room, but it is an option.