I mean, it's first and foremost a murder mystery serialization, but for many long-term fans, there are at most two threads that really matter: first and foremost the black organization that turned Shin'ichi into a child, and secondarily for some the relationship between Shin'ichi and Ran.
For people who literally picked it up for just the murder mystery serialization, obviously nothing feels like filler. For people that want to see the conclusion to the long-running shounen manga that has been going since they were children, 85% of the manga feels like filler. This manga has been going since 2 years before One Piece but has much less reason than One Piece to have not wrapped up its main plot thread by now, especially as Aoyama-sensei ages. Oda is doing his best to tell his complete story whereas many understandably feeling like Aoyama is just milking the property, especially before he started his hiatuses between cases since the average case quality had dropped QUITE a bit back then.
I'm a lot happier with the state of the manga as it is now compared to when he wasn't taking breaks (as long as we get the ending before Aoyama dies, otherwise screw him for procrastinating lol), the hiatuses give Aoyama time to think of more compelling tricks and cases (though he still reuses a few ideas here and there because it's just so long-running lol), but I fully empathize with the people who feel like he's stalled the ending too long. If he wanted to, Aoyama could've decided "no more filler cases" 20 years ago and the manga would've ended in 4 years. He could decide to do the same RIGHT NOW and the manga would end in 1 year. As it is, we're looking like it'll end in another 5 years.