15% is clearly too optimistic. It is closer to 0.15% and it is usually not only about chances but that the person is experiencing extreme suffering despite pain killers that any therapy only prolongs this suffering.
Here is an example: https://www.tecentriq-hcp.com/urothelial-carcinoma/clinical-...
For people with cisplatin (chemo) resistant metastatic (spread through the body) bladder cancer, if you treat with immunotherapy only 6% achieve complete response (cure). The other 94% has a partial response (which means they progressed) or no response at all. So they would likely die of their cancer relatively soon.
Lots of cancer drugs are like that.
Immunotherapy is expensive. Is it worth spending $250,000+ on cancer drugs for a 6% chance of a cure?
Why bother? That's a good question. I think the answer is "because 5% will be cured and another 20% will have some response which means they may live another few years."
But the problem is, you won't know who benefits and who doesn't. So do you deny everyone a chance at a 5% cure of 20% chance of a longer life? No, you treat everyone and see what happens.
The UK government put a price of ~25,000GBP per year of life (quality adjusted). So even if people are cured there is value in extending years of life.
This is a resource allocation problem. People (whether individually or collectively) work to create things of value and direct some percentage of that to medical treatment. If that is wasted in rare cases, that’s unfortunate but we accept some amount of imperfection. If it’s wasted in a lot of cases, many other people will go without needed care who didn’t have to if we allocated that spend more effectively.
How could not money pay part in this? If the continued care costs, let's say $10,000/month (not unusual for the US) and you have 1:1000 chance to live that month, it means this month has cost $10 million dollars. Who is going to pay for that? In fact, taking all this money for yourself, the healthcare for other people will need to be reduced which can lead to shortened life expectation for those people.