Pulling them out of my ass, mostly. Like, the people I know in the field don't seem overly worried, but my own opinion mostly comes from a general awareness that stuff in LEO comes down eventually, and that for the orbit the Starlink Stuff is on, that would probably mean a few years max.
Not my field, and if I actually research it, I might find I'm wrong.
I still maintain that even a complete loss of launch and orbital capability, while of course a great and horrible disaster, wouldn't doom us much more than our current course as a species already is.