In Trump's mind, he'd rather have an incompetent lap dog than the obligatory stubborn 60-something year old who has spent 40 years in DC who will try to talk him out of his policy objectives, which is who he'd have to appoint to replace Hegseth.
Mattis never left active duty in mentality or spirit. Before he resigned over differences of opinion with Trump on deploying ground troops to Syria (with Mattis being strongly on the side of committing forces), he had instituted such policy initiatives as 'thou shalt have 80% readiness, no exceptions' (which magically made stoplight charts green) and randomized carrier deployments.
We need our Secretary of Defense to act like a civilian leader and sometimes accept bad news. It helps if they understand that ships and planes need maintenance cycles.
There was wisdom in requiring someone to be removed from military service for at least 10 years to hold the position.