From Yale University Professor Christine Hayes:

Second of all in this story we see something that we’ll see repeatedly in the Pentateuch, and that is that God has to punt a bit. He has to modify his plans for the first couple, by barring access to the tree of life. That was not something presumably he planned to do. This is in response to, perhaps, their unforeseen disobedience: certainly the way the story unfolds that’s how it seems to us. So despite their newfound mortality, humans are going to be a force to be reckoned with. They’re unpredictable to the very god who created them.