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.
Not too sure if Real Madrid will try to get Mbappe in this transfer window this season. indeed, ancelotti needs an injection of energy for the front line but I suspect that for mbappe they will not maximize the chances of bringing him in this season. Joselu is the main force for next season he will be the player who will replace benzema for next season.
So, for the next winter transfer window, there is a high probability that Real Madrid will take advantage of their strengths to bring in Mbappe and all efforts will probably be carried out for sure in the next winter window.