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.
Sure I assumed that was a given.
I don't think anyone is using these types of sponges for the dishes. If you think of it for what it is (fine sandpaper) the sponge isn't making it very far and I don't think there is much of a risk at all.