God extends Hezekiah's life by 15 years in 2 Kings 20:6. After that, bad things happen to him.
16Then Isaiah said to Hezekiah, “Hear the word of the Lord: 17The time will surely come when everything in your palace, and all that your predecessors have stored up until this day, will be carried off to Babylon. Nothing will be left, says the Lord. 18And some of your descendants, your own flesh and blood who will be born to you, will be taken away, and they will become eunuchs in the palace of the king of Babylon.”
When he dies:
21Hezekiah rested with his ancestors. And Manasseh his son succeeded him as king.
His son is born 3 years after God has extended his life:
2 Kings 21
1Manasseh was twelve years old when he became king, and he reigned in Jerusalem fifty-five years. His mother’s name was Hephzibah. 2He did evil in the eyes of the Lord, following the detestable practices of the nations the Lord had driven out before the Israelites. 3He rebuilt the high places his father Hezekiah had destroyed; he also erected altars to Baal and made an Asherah pole, as Ahab king of Israel had done. He bowed down to all the starry hosts and worshiped them.
Wouldn't Hezekiah be better off if God has not extended his life by 15 years?