20-Year Prospect
The field of oncology is likely to grow rapidly over the next several years, meaning this job is as likely to become obsolete as puppies are to be determined "no longer cute." Oncologists might not have the "aww" appeal of puppies, but we're sure they'll be needed for a long time.
There are so many forms of cancer, no single answer is going to wipe out all of them. In other words, we're not expecting a cure in the next few years that'll force oncologists to have to start giving square dancing lessons just to buy food.
So, don't fear taking all that medical training for nothing. You'll still be needed after the next couple of decades have passed. Hopefully by then cancer treatments will have advanced to the point where it's "monitor and manage," rather than "let's throw everything we have at it and see what happens." The past twenty years have brought remarkable progress in treating the big C. The next twenty promise to be even better.