Since episode 279, the One Piece anime doesn't have ending songs. Instead, it has long openings: almost 3 minutes, instead of the usual 1:30 openings.
What was the reason for this change?
I don't think any official explanations were given (I tried to search but there are only speculations).
I think the most probable reason is the fact that when you're watching broadcasts on TV, as soon as the ending theme song pops up you change to another channel. Almost nobody watches the ending credits/openings and this is true also for other things. Think about when you go to the movies: when the credits starts rolling, is there someone that will stay sit there? Maybe, but that's quite rare. Same for movies on TV, you usually change it because... you really don't care, unless you need to look up some actor/character in those rare occasions.
For this reason, and I'm quite convinced this is the case, they cut the ending theme song, made the opening longer and put the credits there instead. More people are likely to watch it, not just those that like the song, but also once you get to the channel, you're less likely to change it just because it hasn't started yet.
Along with the recap, this makes the "opening part" much longer, even cutting from the usual ~24 minutes of actual episode time.
There could be lots of reasons they did that, the most logical being that it was a production decision, they'll rather pay royalties or have one song made than two, less time and money spent on one 3 min song than two shorter ones.
It's to increase ads revenue and reduce production cost at the same time.
Historically, this happened at the same time when the timeslot for One Piece on Fuji TV was moved from Sunday 19:00 JST (Golden Time, the Japanese equivalent to prime time) to Sunday 9:30 JST (Local Sales Time -- timeslot for procuring program sponsors at each broadcast stations [Japanese Wikipedia]) on October 2006. The reason for moving the timeslot was said due to the removal of anime timeslot during Golden Time and replaced by variety shows instead (in-effect until now).
It also went into some structural changes:
However, since the duration of the main content (A & B part) didn't change, the remaining time was added for CM. Also, since the ED removal, the main part starts around 9:35:40, so there were around 3:10 minutes of CM (minus the OP) before the viewers can enjoy the story. It's also said that the CM between A & B part was also lengthened to more than 3 minutes.
The reason why the duration of the CM was increased was said that it's harder to receive sponsorship on morning timeslot compared to Golden Time (less viewer → fewer ads revenue)
They simply wanted to change things up because One Piece had been on the air for so long. The theme songs are meant to be catchy little jingles by famous artists in Japan. One Piece had basically surpassed a whole generation of people, so there is really no need for two anymore.
They need a longer opening sequence to either re-familiarize long term viewers or new ones who hadn't seen the first 500 episodes. One Piece started simulcasting around "Thriller Bark" arc, as mentioned most people don't sit around to watch. Maybe if it was on actual Japanese TV where there's another show they want to watch next, but not with simulcasting.
And finally, there had been too much filler, so they tested out a huge anniversary opening before making this change. During this anniversary event, they remade the classic "We Are" song, animated all the cover stories. This marked the beginning of the change and there hasn't been an ED since. The way opening and ending credits are handled is also 100% artistic.
Take Supernatural for instance, whose opening credits basically consist of what happened so far mixed with music in a huge montage of previous episodes then throwing you right into the action, or sleepy hollow whose opening credits don't happen until 15 minutes into the episode.
The whole thing is practicality and art. There's no actual rules that say this is how OP and ED should go. As long as the actors are credited, that's all that matters.
I suggest you look up some different shows and see how OP and ED are handled, like some different ones.
You'd be surprised how versatile it all is, especially in other countries (such as Korean dramas, which don't technically have OP and ED as far as i can tell)