They won’t be stumped by a question from a smart undergrad. The best teachers might fail to make it a teachable moment.
Why would I need a world class researcher for e.g. linear algebra? Lots of students struggle with it because of bad teaching, not because it's difficult to understand.
Who would you rather be taught by, someone who spends the rest of the week using what you are being taught, or someone interested in something else (at best, teaching itself, at worst, doing anything else)?
I might suspect bad linear algebra teaching is caused by a lack of interest in linear algebra by the teacher.
Having seen world-class researchers up close, and having experienced their teaching first-hand over many years, I don't believe that being one is any guarantee of also being the other.