And, I’d be fine revoking everyone’s clearance one year after leaving government service.
This over-simplifies how clearances works and is simply not practical.
Clearances aren't just held by direct government employees, but also by contractors. And contracts end and people shift jobs in the real world. Just because someone has for the moment left government service doesn't mean they won't be resuming government service later, in the same or a different capacity. In fact that is very common.
Getting a clearance is the result of an investigation, a gathering of information about the individual that is expensive.
Thus clearances are ultimately associated with _people_. Not who they work for. And this makes perfect sense. Whether someone is trustworthy is not based on who they work for.
What you may be thinking about, which DOES have a lot to do with who you are working for and what you are working on is "need to know". That is a separate but important distinction from 'clearances'.
Having clearances is an aspect of a person -- it establishes level of trustworthiness.
But whether they have access
to any particular information still has to depend not just on their trustworthiness, but also on their need to know
In normal procedures, when someone is "off boarded" from a program, they are debriefed and no longer have a "need to know". Thus while they may (usually) still have their clearances, they no longer have access
Even before Trump pulled this maneuver, it isn't as if Brenner could have just walked into any top secret facility and gotten access to any top secret document that he wanted. He would have had to been approved for access based on a need to know.
Trump pulling this maneuver is pure capriciousness. It doesn't affect Brenner's access to classified information right now (because he probably doesn't have any based on "need to know"). It potentially, however, impacts his future employment opportunities in a big way. So it is very much punitive.