No, we're not. The reason being that sakuga specifically exists because it's trying to utilize the budget for complex designs while relying on cheaper, limited animation in the other episodes in order to focus the talent on specific, story-important moments.
American cartoons tend to have the same amount of budget per episode, and it's often a moderate one to boot. The specially animated scenes tend to be (Not always, but TEND to be) random scenes, not one of narrative significance. For example, the James Baxter White Diamond scene WAS important. But that Studio Trigger guy who did two shots of Stevonnie swiping her sword and kicking a Holo-Pearl was not, but that didn't stop them from giving the animator that worthless scene to animate for.
And finally, these cartoon designs are ALREADY simplified for specific purpose of "making them easier to animate". The idea that they are still cheaply animated except for those guest shots kind of defeats the fucking purpose, doesn't it? At that point, why do we even simplify if we're going to get flat walk cycles and limited movement even with Gravity Falls-tier designs?
We've always had moments in cartoons where some episodes were just spectacularly animated compared to others. This isn't new. If anything, the animation quality has gone down. A random episode of Hey Arnold has more movement and acting than your average episode of like, Amphibia. And I'll post a webm following this post showcasing that.