After this apparent blessing Hezekiah makes a major error through his pride and his punishment is that Judah and his future heirs will be captured by Babylon.
"In those days Hezekiah became ill and was at the point of death. The prophet Isaiah son of Amoz went to him and said, “This is what the Lord says: Put your house in order, because you are going to die; you will not recover.” Hezekiah turned his face to the wall and prayed to the Lord , “Remember, Lord , how I have walked before you faithfully and with wholehearted devotion and have done what is good in your eyes.” And Hezekiah wept bitterly. Before Isaiah had left the middle court, the word of the Lord came to him: “Go back and tell Hezekiah, the ruler of my people, ‘This is what the Lord , the God of your father David, says: I have heard your prayer and seen your tears; I will heal you. On the third day from now you will go up to the temple of the Lord . I will add fifteen years to your life. And I will deliver you and this city from the hand of the king of Assyria. I will defend this city for my sake and for the sake of my servant David.’ ” Then Isaiah said, “Prepare a poultice of figs.” They did so and applied it to the boil, and he recovered."
2 Kings 20:1-7 NIV
"The righteous perish, and no one takes it to heart; the devout are taken away, and no one understands that the righteous are taken away to be spared from evil."
Isaiah 57:1 NIV
These verses together paint such a telling picture about God. It shows us a glimpse of the complete understanding He has about the future and the past and how easy it is for humans with their pinhole lens of the world to misunderstand and judge inaccurately and misconstrue mercy for cruelty or apathy.