In the parable he doesn't merely say, ‘I will arise.’ He does arise, and he goes to his father, and he repents of his sin, and he says, ‘I'm not worthy in the least.
Sadly, there have been many people down the years who have come to the point where they have said in their hearts, ‘I will arise and go to my father, and they have never done it, and they are still lost, and some of them have died that way, and they have gone to everlasting separation from the Lord. What a tragedy it is, because their hearts appear to have been moved. ‘I must repent of my sins’, they have said. ‘I will go to God. I will seek a blessing’, and then they forgot all about it. The next day they changed their minds and they decided to live for this world, and for the things in it, and they never went and they lost their souls.