than they do > than they have
The sentence can be used as written: "than they do" is OK in English.
In this sentence "have" could be seen to be understood, I believe. i.e. "than they do (have)".
than they do > than they have
The sentence can be used as written: "than they do" is OK in English.
In this sentence "have" could be seen to be understood, I believe. i.e. "than they do (have)".