Here's why i think One Piece is awful. And why It would never be able to compete in current manga industry and would have been cancelled and forgotten if it was released today.
Comments that refutes my arguments are welcome.
--Formulaic
Once you've seen a single arc in One piece you practically seen 99% of the show. Its very generic and offers nothing new.
- The good guys go to an island > Crew divides > Meet natives (who are likely subjugated by the evil dictator- the villain) > sad flashbacks > meet villain > villains beat them up and start some time limit doomsday mechanic >crew reunite and each one gets his own villain to beat >crew win> leave island and repeat
Image summarizes it 
Also a ton of interesting stuff get offscreened for some reason.
There is one arc where it slightly freshens up the formula and changes it slightly but it Its still the same thing in the end.
- One if not many fans would agree the best part about it is the scraps of clues that get thrown at us at the end. But this is a disgusting strategy made to keep sunk cost people from leaving.
-Asspulls
In One piece asspulls are many. So i'll only post a few of them
- Gear 5 and Nika Nika Fruit -- This is one of the biggest asspulls in one piece. It was so big that everyone tried to find any justification or some cope about how its foreshadowed. Luffy suddenly has a special power up he pulled out of his ass and His fruit is actually special from the start (the opposite was implied).
- Minky Medicine. Auto Heal asspull
- Haki - Asspull powersystem. It doesn't even make sense retroactively when you apply it to past events.
- Constant powerups -- Every arc has some new undeserved powerup. Oda treats them like candy. Every fight is basically formulaic asspull. You just wait and see what new oda asspull power will give to the good guy.
--Plothole galore. Same as Asspull. Will only be posting some of them
- A weak shark got Shanks arm. Doesn't make any sense when you know how powerful Shanks is.
-Little Depth and generic character archtypes
Oda cannot write deep characters at all. Aside from the exception of characters getting sad when another dies (which is very rare for one piece). How often do one piece characters actually address the loneliness caused from someone death? Their grief? Or how to get better aside from punching harder and eating more meat to grow stronger?
In 1000 chapters there hasn't been any major changes for any character. Luffy is still the loveable hungry goofball generic shonen MC. Zoro is still the angsty edgy badass. Usopp is still the coward guy Sanji is still the horny pervert. Nami is still the greedy girl.
Every character is an archtype and lives within it. even the villain's are one-dimensional archtypes.
This wouldn't be bad if One piece ended in 200 or 100 chapters. But 1000 plus chapters of the same? This is horrible.
--Mystery used to hook people for 1000 of chapters
The Greatest thing about one piece is its mystery. Practically everyone wants to know what the One Piece is. Oda along with the editors skillfully used that to milk more money and sales out of people. What mystery that deserves over 1000 chapters? The reveal will be disappointing no matter what it is.
-Fakeout deaths
Nearly every arc we get some fake out deaths to add artificial tension and stakes. However this backfired since people don't believe a character has died unless Oda confirms it directly.
There is zero danger to the strawhats. They'll survive to the end and they'll get tons of new shiny powerups.
Its impossible to enjoy the fights in this series when there's zero stakes. Even the formulaic "Doomsday clock" always gets stopped at the last second.
-Art and Paneling
This is also one of One Piece biggest flaws. Its nearly unreadable.
see image 
Its also filled with characters reactions. How many times we need to see strawhats eyes popping out of their sockets gawking at luffy?
-Dialogue
Dialogue is also one of the worst things about One Piece. Its like a gag manga or children manga dialogue Its very simple and childish It fits the cartoony aesthetic I guess.
Conclusion: One Piece is a forgettable badly written manga that should have ended 20 years ago. It lived way past its time and its already losing in sales and popularity to other newer better series.
If it was currently running it would have been cancelled within 19 chapters and forgotten. Its carried by nostalgia and peer pressure.