How serious is this?

Despite parkerbot's wishes of death upon the Princess, I think she'll pull through, and I'll tell you why:

1.) She used the words "preventative chemotherapy" and indicated they were doing it to be on the safe side. This suggests that whatever she has, like her father-in-law, the doctors caught it early. She and the Palace press team have not said exactly what kind of cancer it is, so we have no idea.

2.) Before her surgery and the cancer diagnosis, she was already very healthy. Catherine is a very physically active person, eats healthy, and is surrounded by people who love and care about her. She has a very good support system, including her husband, her kids, her parents, her sister, and her friends both in and outside the royal court. She also has fans around the world who are sending her well wishes and praying for her, indicating that she is well loved far beyond the borders of the UK.

3.) She has access to the best doctors in the world, far better than what your average Brit does. Whatever treatment they have for cancer these days, she'll get the best the world has to offer. She might even be given invermectin for treatment, since it has been proven since the 1950s to have cancer-fighting properties.

I think they knew they had to go public eventually. They wanted to take some time first, keep it private for a little while so the family could deal with it.