I don't agree with what they are doing but I don't think there is anything unusual or illegal going on. The elected president or Congress could fire them at any time if they thought that the agency heads were doing anything wrong.
It's wildly undemocratic to push for government employees to usurp the will of the people's representatives.
Like other's have said, the best a government employee can do to protest the direction that the President and Congress set is to resign vocally (as lots of people have done).
Consider the opposite side! People rightly criticize government employees who resist implementing gay rights. But, somehow EPA employees resisting loosening regulations is righteous.
Everyone thinks their political beliefs are righteous.
The point is: rogue government employees should not be exerting their concept of what it morally right in direct conflict with leaders elected by the people.
It is inherently undemocratic. If you want an unelected oligarchy of technocrats, that's something to debate the merits of. But, anyone who values democracy should be appalled by an unelected bureaucrat setting policy against the will of the people.
If you hire / appoint / confirm someone with the motivation of doing something unusual or illegal, you’re hardly going to turn around and fire them for it.
This repeated claim that “things must be fine, otherwise the adults in the room would take action” assumes that there are adults in the room willing or able to take action. At some point it’s nonsensical, and clearly just an attempt to avoid acknowledging the obvious.