So far there is still a chance to sign Mbappe. Madrid have that chance, they might just need to spend a lot of money to get him. The transfer market is still long, Anceloti could have many alternatives if he does not get him. Benzema is indeed no one who can replace. But now that he is gone, Anceloti must use the players he has to compete. So it doesn't matter there will be new periods and new problems when the best players leave.
Although Mbappe already said to stay in PSG, I agree that it is not impossible to sign Mbappe as long as the transfer window is opened. Moreover, Tottenham management already set the price for Harry Kane with an unreasonable price. I heard they want €200m for the transfer fees, it is a too excessive amount of money. For a striker that is almost 30 years old, I'm sure Real Madrid will stop to negotiate for Harry Kane. Now, Real Madrid may focus on Mbappe or Osimhen. Sure, there is still a long time before the end of the transfer window, but Real Madrid has many competitors to sign a top striker like Mbappe and Osimhen. It is not easy to sign a top striker to replace Benzema's role in Real Madrid, I doubt Real Madrid fails to sign a top CF for the next season.