According to Ezekiel, God doesn't just wash the old one and give it back...
"And I will give to them one heart, and a new spirit I will give in their inner parts. And I will remove their heart of stone from their body, and I will give to them a heart of flesh, so that they may walk in my statutes, and they will keep my regulations, and they will do them, and they will be to me a people, and I myself will be to them as God." Ezek 11
And 36 says it even clearer and the reason for it.
"“‘And I will take you from the nations, and I will gather you from all of the lands, and I will bring you to your land. And I will sprinkle on you pure water, and you will be clean from all of your uncleanness, and I will cleanse you from all of your idols. And I will give a new heart to you, and a new spirit I will give into your inner parts, and I will remove the heart of stone from your flesh, and I will give to you a heart of flesh. And I will give my spirit into your inner parts, and I will make it so that you will go in my rules, and my regulations you will remember, and you will do them. And you will dwell in the land that I gave to your ancestors, and you will be to me as a people, and I will be to you as God. And I will save you from all of your uncleanness, and I will call to the grain, and I will cause it to increase, and I will not bring famine upon you. And I will cause the fruit of the tree and the crop of the field to increase, so that you will not suffer again the disgrace of famine among the nations. And you will remember your evil ways and your deeds that were not good, and you will loathe yourself over your iniquities and over your detestable things. But not for your sake am I acting,” declares the Lord Yahweh. “Let it be known to you, be ashamed, and be put to shame because of your ways, house of Israel." Ezek 36
Cuz God wants to! So God will get the glory. When you are saved and when your heart is changed and your life shows it and you are blessed, it is so people will know God and his character. That he would abase himself to save such as you! That he would demean himself to save me! That he would put his seal on us and say, "Mine!" We are given his name, so we can speak honestly of Yahweh, because he has made himself known to us. Not just, God bless, but "My Father, Yahweh did this! He is good!"