Since no one else has gotten into the following aspect of the situation, it probably should be mentioned that it's probably not likely that the DS DVDs, in their present form, anyway, will be produced for decades to come. Video technology has changed extremely rapidly in recent years. There are already two completely different second-generation DVD technologies available as it is (HD DVD and Blu-ray). (Thankfully, the players of both those types are backward compatible with the first-generation technology that produced the DS DVDs.)
Personally, I wouldn't be surprised if the first-generation DVD format doesn't go the way of vinyl LPs in the not all that far distant future. It's also up in the air as to whether or not MPI would upgrade/rerelease the DS episodes on whichever one of the two second-generation formats comes out on top. So, it could be doubtful that MPI would continue producing DS DVDs in their first-generation DVD format for decades to come once the second-generation DVD format takes hold (like VHS killed Beta when it came to videotape). It's possible that we'll see the availability of the DS DVDs in their present form for another decade - certainly on sites like eBay. But, unless MPI does indeed rerelease some sort of upgraded sets, to expect the current sets to be around for decades on sites like Amazon or Deep Discount is probably wishful thinking.