The decrease in incidence of cervical cancer results from better screening methods, and does not reflect any decrease in the number of HPV cases. According to this very informative website, between 250,000 and 1 million cases of cervical dysplasia are diagnosed in the US per year. Dysplasia is a pre-cancerous condition caused by HPV which, left untreated, has a high chance of developing into full-blown cancer.
We aren't just talking about preventing cancer deaths, we're talking about preventing dysplasias which can have serious health consequences. Treatment for dysplasia can be very effective for preventing cancer; unfortunately, it can also be quite effective for preventing future childbirth. Cervical dysplasia may be treated by LEEP, in which part of the cervix is removed. Sometimes, one LEEP procedure doesn't eliminate the virus, and the LEEP must be repeated. Each LEEP procedure damages the cervix, increasing the patient's risk of miscarriage should she get pregnant. Sometimes, the virus penetrates so deeply into the tissue that the only effective treatment is a hysterectomy, leaving the woman sterile. A gynecologist I worked with told me of having to do a hysterectomy on a 19 year old girl who had become infected at age 16, and remained infected after several LEEP treatments.
If just 50% of HPV infections can be prevented with the vaccine, that represents a HUGE savings in terms of treatment costs and overall health benefits.