We will see if Liverpool really made a mistake by selling Sadio Mane. It is still early to make such predictions about the situation. As I said before, it is depending on how the new trio will play and how Darwin Nunez especially will play in this season. Liverpool got really close to a championship in the league last season but lost their chance by inches. They need to put a bigger effort into it this time to be able to get ahead of Manchester City. They will need to decrease the amount of points lost in easier games to make a difference.
Yes you are right, from what I have seen many people are so confident that Darwin Nunez will be able to replace Sadio Mane, in the sense that he will be successful at least Liverpool will not feel the loss of a player who helped them the last few seasons (Sadio Mane). But I myself hope that Nunez will repay that trust with a good performance, to ensure that Liverpool do not regret losing Mane.