One Piece has tension, you people are simply not feeling it.
Experiencing a story is not a one way experience, it's a sharing process : You bring your experience, values, expectations and mindset while the author brings their vision. It is important to understand that if the exchange is corrupted by your own expectations and values of the story, you have a stronger probability to stop liking it.
If you read One Piece thinking it has no tension, you will not feel tension, simply because our mind are structured in such a way that it will negate the obvious and make us focus on what we want to confirm. As such, you will constantly find in One Piece the reasons to confirm this "there is no tension" affirmation. Even if it is not there, even if Oda proves you otherwise like in the last few chapters. That is the power of confirmation bias.
Then, you will reak toxically at people who just started the story and fan who kept the love, dumbfounded by their tensed and surprised reactions in front of the tension.
I personnally feel the tension in One Piece, not simply because I love it, but because I know for a fact that Oda can switch gears at anytime. You simply made yourself believe that - despite the factual evcidences - he would never do that.
Twists after twists, conflicts after conflicts, and probably through a bad reading week to week experience and the idea that you knew better, you detached yourself from the impact of the flow of One Piece.
There are ways to criticize a story ethically, but this is not it. This is a superiority complex.