Adam Clarke on Genesis 6:19

To keep them alive - God might have destroyed all the animal creation, and created others to occupy the new world, but he chose rather to preserve those already created. The Creator and Preserver of the universe does nothing but what is essentially necessary to be done. Nothing should be wantonly wasted; nor should power or skill be lavished where no necessity exists; and yet it required more means and economy to preserve the old than to have created new ones. Such respect has God to the work of his hands, that nothing but what is essential to the credit of his justice and holiness shall ever induce him to destroy any thing he has made.