Thiem would be a grand slam champion already if Nadal wasn't around. 2nd best player on clay the last 2 years, and unlike any other young player(though he is 25) , Has actually done something in a grand slam with a semi-final and final in the last 2 years. Zverev hasn't even made 1 semi yet and 1 quarter(?) for comparison.
Theims issue is that he is a one surface player. he is a 11-20 ranked player on hardcourt.