Humankind and cancer

One Day I was talking to my postdoc mentor about how a therapy for total cancer eradication should look like. We were not arguing any specific strategy bust just were trying to imagine how a cure for cancer would look like and how we would describe it if we found a Djinn in a desert.
The dialog proceeded a bit like this:

Mentor-An average tumor should have something around 5-10 billion cells. This tumor, however, is often spreaded around the body in regions from which it cannot be extracted without causing patient death.
I-But can't we help the body fighting it? Cancer immunotherapy proposes to identify over-expressed proteins that can be targeted by immune system.
M-Do you know the success rate of these therapies?
I-5-10%?
M-You'd be surprised by what a placebo can do...
I-So how could we envisage a therapy? Any sort of cure? Maybe keeping cancer as a chronic disease, just slow down its growth?
M-Coming back to the tumor size, imagine humankind where a tumor on earth, there are around 6 billion people, spreaded all over the world. They eat differently, have different resistant to diseases, live in different weathers, humankind is pretty much as heterogeneous as tumors...and as pernicious to the planet as tumors are to human body.
I-So the problems are equivalent: how to eradicate human kind and save the planet is like eradicating a tumor?

Now how can we overcome such a challenge? The more we proliferate on Earth and the more we pollute, destroy natural ecosystems, the closer we get to our own extinction, but will life on the planet survive our extinction or shall we bring all life with us like a real cancer?