One Piece is good? I assumed it was like every other shounen anime where one or maybe two seasons are good then they string it along forever by doing a plot reset over and over to tell a variation of the same story while foreshadowing something that never arrives. I saw a few episodes back when it originally came out and was turned off by the quality of the animation.
One Piece is very, very good. There are elements of what you've described, arcs are split by islands and the major end goal has always been getting the treasure One Piece which is essentially at the end of the world. But arcs are never a hard reset, rather they constantly build on top of eachother
Where One Piece excels is 1) world building, every single island is unique and so well crafted, everything from the unique flora and fauna, geopolitics, monetary systems is built out. It works so well in making each new island a fresh and exciting adventure. But the longer the series and the more you see the big picture the more you realise Oda has carefully weaved it into a cohesive entire world that is rivalled by no manga and only a few fantasy series
2) Characters - there are over 1100 named characters in the series. It is a huge cast but Oda doesn't waste that, they are all used often in interesting ways to come in and out of the story over time. It creates a world where not just the main characters are changing but the world and characters around them too.
The art style which many complain about at first is also essential to this. After One Piece it makes it hard to go to some other manga where everyone almost looks like the same person
3) Length - as much as length is a barrier to entry, it has also created a world in which Oda plans way ahead and drops hints and references that pay off years later. It makes for a story with a clear vision that isn't being made up on the run. He is genuinely masterful at this and there are examples of things in chapter 1 being hinted at that are only paying off after chapter 1000
If you saw it when it first came out you probably saw the 4kids abomination which tried to turn it into something for 5 year olds. It basically killed the chance of popularity for the show in the West. They turned cigarettes into lollipops and guns into water guns ffs