Why, because Watson gave a bunch of laughably wrong answers? That's not how the mathematician machine would work. It would combine brainstorming (probably via massive parallel statistical search), at which people are currently much better than machines, with rigorous proof checking, at which computers are much better than people.
Still, in 15 years? I'll wager against this prediction.
This answers the question, I believe. Are all human mathematicians in jeopardy? No.
May we need less human mathematicians in the future and maintain a strong mathematical community? Likely.