Oh how cute.You believe Disney movies.
You misunderstand.
Kate is never going to be Queen.
Harry is second in line by blood, he will be King if William is gone.
William interestingly is not missing.
Kate is the Kings wife. She is not even in line for the throne. William dies she fucks off to live with the lover(s) of her choice in comfort on the wealth of the 'Firm' or on the backs of the British taxpayer.