What were they paying to keep these greybeards around?

RAD always paid very well by gamedev standards. But that was never the main reason people worked there.

Some of the engineers are older but Fabian is in his thirties and last I checked his goatee wasn't greying.

AFAIR a public H1B entry from mid-2000 shows $300k salary, which was impressive at the time even for non-gamedev.

Thats insane for wages.

Work environment matters more than pay, and the pay presumably was very good. Imagine being down the hall from the people who literally started indie game development, or demo-scene royalty.

Interesting job with other people they like is probably a good guess.