Ah, Doctor Who! I was only ever a fan of the older shows, and even then was never a big one.
Are we afraid that we'll never get the first 4 seasons after/if DVD dies?
I shouldn't think so. They would probably still be available on Netflix or similar services in the future, and of course preservations would be easy to do. It's things that never had a decent home video release or were released pre-DVD that I worry about losing.