With a max of one million satellites in the constellation, if Space X targets the same 5 year lifespan of its current satellites, that would be 4,000 launches a year.
In 2020, Space X had 25 launches, then 31 in 2021, then 61, then 96, then 136, and last year launched 167. That's an average of around 50% more per year. At that rate, to get to the 24x increase needed to launch faster than the constellation can support, they'll only need to keep on that trajectory for eight more years.
Why is that unsustainable? You need to present an argument that is more than just "large number! wave hands!"
It's just like the arguments used by anti-renewable energy forces here on Earth. "Large number (of PV modules and wind turbines and batteries)! Unpossible!"
And no, that launch rate does not "wreck the planet".
The most significant limitation on Starship launches is from injection of water into the stratosphere and mesosphere, but 20,000 launches is far away from that limit.
In 2020, Space X had 25 launches, then 31 in 2021, then 61, then 96, then 136, and last year launched 167. That's an average of around 50% more per year. At that rate, to get to the 24x increase needed to launch faster than the constellation can support, they'll only need to keep on that trajectory for eight more years.