How difficult would it be for a Democratic POTUS to replace Trump's executive branch employees?

“Competence” is not the word I’d use to describe Obama’s appointees, and I have no reason to expect any more “competence” from the next Dem’s nominees than the last one’s. As for “petty spitefulness”, it’s about teaching the Dems that their actions have consequences. You can’t vote in virtual lockstep against Republican nominees and not expect them to respond in kind.