I think including these bonus episodes is great! I don't see it in any way as meaning future seasons won't be released. As with all other season sets, if it sells we'll get more, and I think this set will sell quite well!
I can't imagine it not being in season sets, so I'm not really worried about that. I'm really glad they didn't wait until the new movie comes out (is that still being made?). This is one of maybe a handful of shows on my wish list that hadn't been announced yet!