If you think the parable is only about the mercy of God you have made a mistake. It is certainly about the mercy of God. But there is much more. You have missed some of the point.
To repeat. The parable is an answer to the question why does Jesus eat with sinners? The parable is about a banquet which both sons are invited to attend. The Lord in the parable invites both his sons to eat. But only one of them comes. The older son does not come. The elder son does not want to be a part of the banquet.
In the scriptures the banquet is often a sign of Heaven. The elder son refuses to enter the banquet. The younger son accepts the invitation to the banquet. Jesus is a great teacher. If you think the oldest son got a bad deal, you have missed the point of the parable. We all know that the youngest son got a great deal from the mercy of his father. But the Parable is equally about the eldest son. His resentment and unforgiveness of his brother leads to his refusal to attend the banquet.
Why does Jesus eat with sinners? The answer is this. Jesus eats with everyone who is invited to the banquet. The price of entry is forgiveness.