As much as you'd save on not having a voice actor, I'm pretty sure a modern cartoon or anime that doesn't have a protagonist who speaks wouldn't do as well as it did in the past when you had the real old cartoons that came out before sound came with movies and you didn't need a live orchestra.

I can't speak for anyone else, but I'd be get kinds bored after a few episodes of not hearing the protagonist speak at all. Closest I can think of that we've gotten is probably the remake of Dororo that came out somewhere close to a half decade ago. And that's with a protagonist who only really started speaking gradually more as the series progressed.