A very-general rule of thumb some (not all) oncologists use is to avoid aggressive cancer interventions unless the patient would otherwise expect at least 10 years of reasonable-quality life. If your dad had outstanding health before the cancer, and had had a reasonable hope of living to 88 or beyond, then major interventions might make sense; if he already had serious comorbidities (like heart or kidney problems) that made that less likely, then aggressive interventions might just take away some of the good remaining years with the people he loves.

The alternative to aggressive treatments, so-called "palliative" care, just means lower doses and gentler treatments aimed at keeping him comfortable, but still slowing down the disease's progression where it's possible without tough side-effects.

It's not easy talking about this kind of thing, but it sounds like your dad knows what he's doing, and it's good you're supporting his decision. ❤️