get paid severance if they were fired - it's likely them "retiring" saves the company money overall.

Typically that is true but at this level executives have contracts with non-compete clauses and as part of that even voluntary departures usually come with a severence, since they aren't allowed to work in the industry for 6-12 months after leaving (unless they negotiate something as part of their departure). It's very likely he got a generous payout.

It's seen as a necessity for protecting intellectual property and company knowledge that the leaders take with them when they leave. It's why so many execs start their own businesses after leaving big companies but don't officially open shop for a while.