Is he the greatest singer of all time, no. Is he the greatest musician chops-wise of all time, no. Greatest songwriter, no. However, he is damn good at all these and has an incredible body of work to back it up, in many different genres/styles. Damn straight he's a "musician's musician." You'd be hard pressed to find any serious musician, from country to progressive death metal, who didn't admire Billy Joel. Plus, those legendary encounters where he had the kid play piano for him onstage and where he showed up to a grade school performing his songs and just chilled in the audience...goes a long way in my book.
When it's all said and done, he's in the upper echelon for sure, yes. I am really struggling to find an artist to compare him to in terms of versatility, longevity, commercial success, etc.