It's a pretty natural conclusion from many economics teachings that will highlight money only has value in the middle of a transaction and is virtually ethereal while sitting in any account.
It reduces the multiplier on money supply, which can be harmful in aggregate if it is sufficient to create a deflationary monetary environment. Given how little impact a single earner who isn't Jeff Bezos can make, or even Bezos himself in the face of offsets from central banks, I highly doubt any individual's savings efforts are doing any harm in practice.
You may as well say that taking any water out of the ocean is immoral because if all water was removed, the fish would die.
