It is a lot of money to pay for a player that will be on the bench. However he is only 20, so it could be seen as an investment into the future. Messi and Suarez are both 30 and while neither is going to retire any time soon that is around the age attacking players begin to decline. So for this season they will be playing regularly with Dembele on the bench, but over 2-3 years his game time will steadily increase until he eventually takes over as being the starter.