The remaining Velvia emulsions may be around for a few years yet, but the E6 process required to bring the images to life is diminishing in availability, save for home users. This in my eyes is the major threat, not the availability of E6 films. Once the process goes, demand for E6 film will I imagine drop very sharply and no amount of hording or insurance against "discontinuance" will reverse the problem. Summary: use Velvia now while E6 processing is reasonably easy to obtain and enjoy the results.
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.