if a 100% effective cure (or preventative) for cancer comes around, what will be the next big goal?

Making the cure accessible. Some cancer treatments are more effective AND much more expensive. Companies may be looking for return on investment, or the treatment is tailor-made from the patient's own cells, and inherently labor intensive and hard to produce.

Either way, curing 100% of the richest 5% who can afford it isn't the most satifying outcome.