The key to making sure E6 is available long term, making sure you have a good stash of the CD-4 colour developing agent, the rest of the process is simply ordinary chemicals you can get from most bulk chemical suppliers. When E6 processing is no longer available, you just have to roll your own. A bottle of CD4, and a 20 year supply of film in the freezer is going to last a lifetime for many people.
Wouldn't the problem be the long-term (20 years) conservation of CD4? I would be very glad if I could invest into a 20-years slide stock, I have always been worried about long-term preservation of chemicals.