There are people on HN far better qualified than I to discuss both orbital mechanics and spacecraft safety assessments but I'll give it a layman's stab based purely on the high-level concepts (which is all I know).

They know there's little to no risk to aircraft or people hundreds or thousands of miles to the East of a Starship RUD in orbit because they know exactly what's inside Starship and how it's built. They model how it will break up when traveling at these insane speeds and how the metal masses will melt and burn up during re-entry. They actually test this stuff in blast furnaces. It's a statistical model so it's theoretically possible a few small bits could make it to the ground on rare occasion, so we can't say debris will never happen - but there's been a lot of history and testing and the experts are confident it's extremely safe.

The case of the MIR space station was very different than a Starship. MIR was built a long time ago by the Soviet Union and they used a big, heavily shielded power plant. That lead shielding was really the part that had a significant risk of not burning up fully on re-entry. Starship, Starlink satellites and other modern spacecraft are now usually designed to burn up on reentry. However, there are still some things in orbit and things we'll need to put in orbit in the future that won't entirely burn up on reentry. There will always be a very small risk of an accidental uncontrolled reentry causing a threat. However, these risks are vanishingly small both because we design these spacecraft with redundant systems and fail-safes and because Earth is mostly uninhabited oceans, much of our landmasses are unpopulated or sparely populated, even in the unlikely event one of the few spacecraft with a large mass that won't entirely burn up has failed and is de-orbiting out of control, we can still blow it up - and timing that at the right moment will still put it down in a safe place (like it did with MIR). There's no such thing as absolute 100% perfect safety. But you're far, far more likely to die from a great white shark attack than be injured by satellite debris.

More to the point, a huge number of meteorites hit Earth every year and it's estimated over 17,000 survive to hit the surface. There are a bunch listed right now on eBay. Do you know anyone injured by any of the 17,000 space rocks that crashed into our planet this year or any airliners hit by one?

That description of heavy lead shielding of a power plant on Mir surprises me since photos show it as having solar arrays. Wikipedia also gives the power source as solar with no mention of lead components. Can you add further details of this?