Keeping h1b sponsorship is likely one big one. I hope the government entity that watches over compliance with that status makes sure the Musk doesn't abuse their situation :(

People with medical issues who can't afford to be without insurance.

The desperate stay.

the competent 10%?

just joking, bootstrap aside the only thing I remember about Twitter engineers was the "happiness" password incident and several similar cases. Competent may be a stretch.