If we wanted patients to survive long term, then maybe we could try a treatment that doesn't destroy their immune system in the process.

Invent it and your grandchildren will retire rich.

The most obvious, naive approach is banking blood & marrow prior to treatment. However, there's a need to clear metastatic cells (CTCs) or train the immune system to find and kill them so that it doesn't reintroduce CTCs upon retransfusion.