I agree with you that there will never be a universal cure for cancer after the person is diagnosed with cancer. The problem is not that each cancer is unique (in fact there are many common patterns and stereotyped responses to treatments) the problem is that cancer is difficult to eradicate once established. Trying to cure advanced cancer is the wrong idea, most probably, despite colossal resources deployed towards that aim. Even curing an early stage cancer is a fraught process, and some apparently cured cancers can sit dormant for 20 or 30 years before returning to be incurable.
However, I believe that there will eventually be a strategy which prevents almost all cancers. Cancer begins from one cell 100% of the time, and eliminating that cell or preventing the transformation in the first place will stop the cancer from ever developing. One way is to genetically modify the human organism to be cancer resistant. We know this is possible in theory, because there are organisms out there which seem quite resistant to cancers for their size (the naked mole rat, elephants, the bowhead whale). But rationally altering the genome of everyone on the planet is a long way off and possibly unpalatable to many. There are other more near term possibilities.
I should probably add that cancer research is my day job.