A million satellites isn't going to be 100 tons; even if they're all on the small side, say 100 kg each, the total is 100,000 tons, therefore by your numbers if they last on orbit for 3 years they'd double to triple the mass rate burning up on aero entry. I think SpaceX actually talking about 1-10 tons/satellite making this more like 10-100x if they last 3 years, but between AI hallucinations getting and Musk's increasing disconnect from reality (let alone political toxicity) this is basically irrelevant. SpaceX won't reach these higher masses to orbit spread over this number of satellites regardless.

Aggravatingly, I have seen research estimating that even the much smaller number of satellites currently in orbit is already enough to be unstable with regard to a Kessler cascade, and any question about the realism of Musk's goals from finance and engineering limits is clearly not enough to prevent this kind of scenario. Which may result in other governments interfering with his ketamine supply to make sure their satellites aren't caught up in one.

Simplest helpful thing for the Kessler problem is "just"* have fewer larger satellites, and if Starship actually delivers the launch costs necessary to make space-based data centres worth the bother vs. just buying some cheap desert land, I anticipate Musk getting managed upwards by his staff in this regard.

* nothing in space is "just"