If you are an English patriot, this is the nightmare scenario: Mary Tudor dies shortly after giving birth to a healthy son. The ensuing government is dominated by Philip and the Spaniards. A Protestant rebellion is crushed by Spanish troops. The Inquisition comes to England. The Dutch revolt is overcome with English assistance. England is now effectively a Spanish colony, and the Counter-Reformation is triumphant all over Europe.
It could have happened. What appears inevitable today is largely the product of luck.