For my friend specifically, staying on the IC track, becoming a Staff engineer and switching companies would have given him ~20-30% more than the EM promotion he was offered.

This is missing something... the friend wouldn't immediately become a staff engineer - that could take just as long, or longer, than a promotion to middle management.

At least where I am, the staff engineer equivalent (called Technical Fellow here) is considered Director or VP equivalent. In an engineering org of thousands, we have tens of these positions.

Or, if I've misjudged what "staff engineer" means, our next lower position would be principal engineer (typically 1 in 10-15 engineering ICs, roughly). And their salaries are in the ballpark of our engineering managers.

Anyway, all this sort of misses the point - it's two completely different jobs. I know plenty of people who don't want to manage people. Or tried and hated it. And plenty of people who are bored with coding and want a chance to put their management skills to work.

EDIT - grabbed this from another comment... - L1: Intern with undergrad degree - L2: Intern with graduate degree - L3: Junior - L4: Intermediate - L5: Senior - L6: Staff - L7: Senior Staff - L8: Principal - L9: Distinguished - L10: Fellow

We have fewer levels than that... - Engineering Intern - Engineer 1 - Engineer 2 - Senior Engineer - Lead Engineer - Principal - Senior Principal - Tech Fellow

So, staff is somewhere close to our Lead or Principal, who earn similar money as line managers. And only Principal+ are on a bonus plan (where all people managers are). For any of the lower ICs, a bonus is a rare thing (where for higher positions and managers, it's part of the comp package).