Here are some facts:
It has been 15 years since Alabasta Arc was finished.
Although there's a movie which was adapted from Alabasta Arc, it was released 10 years ago. And the movie was heavily rushed because of many reasons including schedule issues, not to mention that there are not many people who've watched this movie.
Unlike oversea audiences who can easily watch the anime again and again, Japanese audiences cannot access episodes which was released 15 years ago as easily as oversea audiences can do. In this case, many OP fans didn't have chance to re-watch these classical moments while they do miss the story and characters a lot.
Quite a number of audiences even don't know what happened in the past and who is Vivi at all because they began to watch One Piece much later.
Now I think we can easily get why they included many classical moments of Alabasta Arc based on the facts I mentioned above: indeed these two episodes (#777 and #778) are two special episodes with an "episode-of" style. By the way, the episode director of #777 is the director of Episode of Nami and Episode of Merry.
Production wise it also had a special-episode standard. This episode was apparently one of the most expensive and highest priority episode in these few years, in every aspects. For example, many good even great animators joined this episode, and all the scenes of Alabasta Arc were remasted from beginning. The overall quality of these remasted scenes were overall A grade or even better. (By the way, they didn't need to do this hard if they just wanted to "drag out" the anime.)
I have to say that I may feel very upset If one thought this episode was bad because of these remasted flashbacks of Alabasta.
There were still some problems here and there in Zou Arc, but I believe that many audiences has already noticed the progress that the anime staff has made. And this episode and #775 has already shown that the staff are trying to do something.
–- Update From New Post Merge ---
Then sign me up for a filler arc. At the very least they would have the decency to place it between arcs, while the straws are travelling.
In the case of Toei that's pretty much exactly what it does. They've proven they can't work an even close to decent pacing with a whole chapter, how the hell are they going to make it work with a half? Like this episode was around 5 minutes of original content. I feel really bad for the poor animators with a vision and a hope to really put their watermark out there and then they are just stiffled by moneygrubbing executives who don't give a shit about making good art, but still rake in the most cash. It's pretty scumbaggy.
If you think this episode was around only "five" minutes of original content, I think maybe you should have a check again. And I have written some words that why they included these flashbacks.
By the way, I think they have proven that they can of course make an episode with over all more than decent pacing when it covers a full chapter.