It's not difficult to define what curing cancer is, we've got tonnes of measures we can define. Let's define curing breast cancer - we have cured breast cancer if the 5 year survival rate is above 99% for all SEER stages, or less than 1 in 1000 women get diagnosed with TNM stage 1 or above. It's not that hard, it's just that deep down, they don't want to be judged on that basis.
(Details of the definition of these terms: https://www.cancer.org/cancer/types/breast-cancer/understand...)