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Having a group of specially skilled, highly trained people betraying your interests to work against you sounds like it would be worse than having a less skilled less trained force that is actually loyal.
That he wanted the dude fired isn't really newsworthy, but how he learned about the usher meeting with Biden is. The Secret Service told him. That means at least one of the Secret Service agents assigned to protect Biden at that time was sharing information he learned while on the job with Trump's people. No wonder there have been reports of Biden being careful of what he says around the Secret Service. >

I still don’t understand how Biden hasn’t cleared out the SS. >

It’s a group of very specially skilled, highly trained people. They can’t easily replace the entire detail and even if he did with agents from other areas, there’s no telling that would be any better.
His only other good option may be a military detail and that comes with its own host of issues.