Moh's hardness scale whats a word with you.

That's only true if they're less hard than whatever this is.

I use these sponges to get food off plates. They are definitely not harder than the plate and are not ruining it. Nor are they ruining stainless steel.

Maybe wood varnish is what you're refering to, or pain on a wall, etc.

As far as the microplastics angle, yeah, these things just literally disintegrate along with the crap, particularly oils, they pick up. That's why they're so useful. Add some soap to them and they get even better at washing.