Along these lines, I tend to agree with Netty that usually a company doesn't discontinue something unless it is a poor seller. (If you were satisfied with sales, why would you discontinue it?) Perhaps the WDTs were not meeting expectations and they felt a new launch with a fresh packaging angle was the way to revive the WDT series.
Disney merchandising is a little different than a lot of products. They have a huge catalog of media, that has had a very long lifetime, across many formats. Think of one short being presented, from theater ticket sales, to 16 mm home video projector, to tv shows (clips on mickey mouse club, WWOD, disney channel, etc. - multiple tv shows), to vhs tapes (often in multiple versions - clipped as sing along songs, or on different collections), to dvd to blue ray. And I'm probably not thinking of something there. It is also marketed primarily to families with children. Yes, there are "collectors" and people who really like this material, but the truth is the mass market is families with children. These families age. So there are "waves" offered periodically to new families with children. When I was a kid, this meant an older movie might be re-released at the theater. Or it would be shown on Saturday night's world of disney. Years later that same movie might be shown on disney channel to a new group of children! Or sold on vhs, etc. This is why the movies would "go back in to the vault." Take it off the market a few years and there will be a whole new crop of families with children to buy it.
I think this is why they got rid of vault disney on tv. In a world of tivo and dvd burners, there was more money in selling people dvds. These were the treasures. Maybe that line has run its course and they will reformat them and sell them again in a different way. That is the way the disney company works. If you think they have some sort of obligation to keep a set going, well, that really doesn't seem to mean much.
I'll give you an example. Back in the 1990s I would be the classic disney music cds. Before that series, there was a box set of three cds that had classic disney songs on it. I had some friends that had that set, but I never did. That was when cd players were pretty new. Then later they came out with the disney classic cds. I have two sets now actually, of volume 1-5. Here is basically what they look like:
Basiccally, every few years one of these cds would come out with a lot of songs from the more recent movies (for example, the last one has nightmare before christmas and hunchback songs) along with a lot of classic songs. There are still PLENTY of classic songs that aren't on these five cds. Actually, some of the sing along songs vhs have songs not on these So a few years ago, there had been enough time since the last one and I was looking forward to a volume six with stuff like toy story and Tarzan, etc. But wait. Probably about five years ago one day I was in the disney store and was shocked to see a whole new cd like this:
Well, sure enough, there was some tarzan songs. But they also started from scratch and double dipped from the songs that had been in those five cds from the series they had been releasing over the previous several years! Instead of making a volume six, they started a new series at volume one.
Well, I never bought a volume of that new set, because I was so burned feeling. But I'm sure a lot of people did. I'm sure that there were a lot of people who weren't buying cds when that first set was around who went in to the store and there was a set of classic disney songs.
Let's see. The Tarzan dvd followed by the Tarzan special edition a while later. I think we have four versions of mary poppins in our house between vhs and dvd. I actually took the first alice in wonderland dvd (that I had never opened) back to the disney store and traded it for the two disc one. There have been new dvd versions of robin hood, dumbo, fox and hound. Lots of double dipping. I recently pre-ordered another version of peter pan. The truth is, between dvd and vhs we completed a collection of the classic animation some time ago, so it is something of a surprise that I we are still buying dvds I guess. But there you have it.
I guess what I am saying is, it really seems to me like there should at least be completion of the donald duck treasures. Actually, those were the treasures I was most looking forward to, partly so that I could the last couple of humphrey bear shorts on dvd.
But the truth is, I have seen nothing from disney marketing in my life that gives me any reason to believe that they would ever "complete the set" if they decide to package the shorts in some other way. Actually, that would just seem par for the course.
Very effective! 
I am just kidding!