Don’t buy your logic. Way back it might have applied, but now the left welcomes the new face to throw more unfounded accusations toward. Barr, as a seasoned professional, doesn’t take the left too seriously, which pisses them off even more (love the handcuffs joke he put on NP).
Trump gave AG to Sessions as payback for backing Trump when most of official DC wouldn’t stop laughing. Trump regretted that decision with Sessions recusal (or that was the script). Whitaker was the useful intermediate temp,as you say, and by the time Trump chose Barr, he knew what qualities he wanted/needed. Score one for quick learning on POTUS’s part.
The left has absolutely no problem trying to take down Barr, but his legal intellect turns the tables on them every time. They are scared.