Edit: I should note this all comes from the perspective of one tricking sett in high elo (masters+) for a couple years
Over time riot keeps exclusively putting power in setts W, they have forced his power budget into a single ability to apeal to lower elos without actually addressing the issues that make him weak
Even when you look at mid Elo's sett's win rate consistently falls lower as you encounter better kiting and peel through the ranks, the game sett was designed around has shifted and become more mobile over time.
And this doesn't just effect good players, it's baked into the system. An example is with the addition of t3 boots on mid quest, sett has become unable to stick on even the more immobile mages. even with the Additional help of stride breaker, an item sett is forced to build every game.
Another thing is his build, his stream lined power budget leads to a lack of any build variety for the champ. Check every good players build and it' always the same 4 items.
stride breaker, black cleaver, overlords bloodmail and steraks.
Forcing his W to be only part of his kit worth building for, forces him to build the same combination of move speed, ad and hp every single game. He has very little breathing room to build the stronger figher items that the rest of the champions in his class get access too.
His forced into weak itemization to play for an ability that severely limits his versatility in fights.
His W in my opinion does too much damage, it's eating the power budget of sett as a champion. Causing him to have less mobility and damage on Q. Causing him to rely on his higher Cooldown abilities, especially his E.
Sett's E has become an ability exclusively used to land W damage, it's become less viable to use it for gap close or damage because his W is so important. the rest of his kits play style then becomes about how to best get value out of a single ability.
He doesn't get to have catch out in his Q or E because it would make his W too easy to hit. Removing power from W would allow Sett to have lower Cooldown E's as him getting to use 2 in a fight is less of an issue.
It could also allow Sett to have more mobility on Q. And let's compare sett Q to other bruisers like Garen or Volibear, both their Q abilities have more move speed, omni directional move speed and longer movespeed duration. Garen even gets a slow cleanse on his while already have tenacity on his W.
All this utility that lets the champions function in modern league that sett doesn't get access too.