The President had no way to know who was trustworthy when he came in. He can’t fire everyone because someone needs to do the job. So I am sure they had some testing in there to see who is a partisan hack and who is a patriot. Party is immaterial.
It’s also important to remember that as despicable as we think their actions on the job were, hating the president and being vocal about it is not a crime (if it was, lots of our friends would have been fired during the Obamanation). They are federal employees who are protected by not only all employment law but the Civil Service protections which are probably the strongest (for the employee) anywhere. We all know that what would get you fired in a private company would not even justify a reprimand in the government.
It’s the height of naivety to think Trump could or would have just fired everyone when he came on the job. All that would have done is bury him under a mountain of wrongful termination lawsuits, which he would have lost.
Even in the private sector, the first thing HR will tell you if you want to try to fire someone is “document everything”, and they’ll want at least 6 months worth of documentation including all attempts to correct the performance deficit. When you have swamp critters working for a swamp boss, who’s gonna do that?