Perhaps immune-based therapies like CAR-T are based on the premise that there are many cancerous cells in your body all the time, but your immune system deals with them, and it’s only when it fails to do so that you end up in the pathological state. So the “single cure” is the normally-functioning immune system?
That might be part of it. And yet sometimes people with normally-functioning immune systems also get cancer. So while that might be an effective treatment for some patients it's not going to be a universal cure.
There is no free lunch in biology. Augmenting the immune system to better attack cancer is going to cause other problems. It's so naive to think there is some simple solution that will improve on a billion years of evolution. I mean it's not impossible but realistically what are the odds?
There won't be any magic for cancer. It's just going to be slow grind to solve one hard problem after another.
There is no free lunch outside biology either. The problems that come with stronger immune systems may be more tractable or at least less unpleasant than cancer.
Also, you seem to be very pessimistic. Many interventions in the history of medicine, like washing hands or the first vaccine against smallpox, were almost "magical" in their efficiency: they addressed a lot of problems through a relatively trivial intervention.
It is likely that a lot of this low-hanging fruit has been picked up, but you insinuate that there isn't any low-hanging fruit to begin with, only an endless slog of attacking hard problems. That is way too negative.
