The economy's vitality depends to a fargreater extent than most Americans realize on the fortunes of tiny shops and restaurants, neigh-borhood services and factories.

对于黑体部分，在书中关于than的分析：该从句的主语是most Americans,谓语是realize，宾语是than，指代extent。

但是than词性是介词或者连词，怎么可以作宾语指代extent呢？