I get what you mean. Let me try to explain my point of view better.
On the first PW you racked up hundreds of enemies. There was even an achievement about getting 1000 KOs on a stage (that was imposible to get without buying the DLC, trust me I tried), so yeah, getting a 1000 KOs on that game was a big thing. Yet, the game sucked. It was fun enough at the time, since it was the only OP musou experience, but it was very bland. The stages were divided into squares, the objectives were boring and predictable, and the system overall had plenty of corners to round.
Now let's look at PW3, it's normal to get at least a 1000 KOs on any stage and in fact it's kinda the bare minimum to get a decent score. So the Get a 1000 KOs achievement is an easy one instead of an impossible one, and imo they should've had an harder achieve like Get 2500 KOs or something, the same way you have an achieve about getting 300 KOs with one Kizuna Rush.
The stages have tons of variations, despite not every stage being a winner. On story mode you have several fixed side missions and half of them can't be completed in one go, you'll probably need to do the level with different characters or take different routes and by doing so having a different experience every time. On some stages you can do everything in one go, but you'll need to have leveled up your character and it's a challenge.
On the Free Mode, you have fixed main objectives, on some stages you actually have to choose which side you're fighting on before you start (meaning then you can repeat the stage, choose the opposite side and have a different experience), and then half a douzen of random side missions which will pop up in the middle of the level and give you a different experience every single time.
Yes it's still limited after you pick up some patterns but when you choose a level play to you're getting to choose which side you're on, the stage map is gonna be random, the side missions will be random, and the characters on those side missions will also be random. You could spend all day replaying the same level and get different things every single time.
In PW4? Well, if you get a random rowdy Luffy as an enemy it's going to actually be Brulle in a disguise. And yes, she could be disguised as any character in the game, since she doesn't even have a character model, just a portrait and a few voiced lines. But no, it's always the same side-mission taken from the story mode over and over again. If a random Luffy shows up on the stage you know it's Brulle and that is boring as hell.
PW4 has a dash button meant to help just advance in the stage and ignore the mooks, so you can focus on the fun part, getting your objectives and missions, but what's the point if those objectives and missions are always the same boring ones?