The vaporized materials themselves are a problem. When we're building these mega-constellations, we're putting some real mass up there. We're introducing all sorts of exotic materials into the stratosphere that would not naturally occur there at those concentrations. And remember, this is a very sensitive environment. The actual volume of CFCs we introduced into the stratosphere wasn't that large. The volume of all our AC refrigerant and hair spray cans was nothing compared to the atmosphere. We may actually not be that far from the sheer volume of satellites affecting the ozone layer as they decay.

The stratosphere is an environment like any other. It has a finite ability to absorb and process any form of pollution without noticeable and significant effects. I'm not qualified enough to estimate the number of satellite reentries to damage the ozone layer or to have other deleterious effects, but at least from that study featured in that video, we may not be far off. The story of civilization has been repeatedly realizing that what we once considered infinite dumping grounds were anything but. And the stratosphere is no different.

Edit: may have misinterpreted parent comment and went off on a wild tangent.

Due to the physics and orbital mechanics, near impossible thankfully

Most of the energy from the rocket that launched the satellite went into pushing it fast sideways. If you wanted to slow it down enough to drop straight down, you'd just about need a second rocket to stop it.