If Real Madrid fail to sign Mbappe, then surely a contract with Kane will be signed. Moreover, there was information recently that Kane was telling his Tottenham teammates that he would move to Real Madrid. Real Madrid, after Benzema's departure, needs a super scoring striker with experience. In addition, recently, not least after working with Mourinho, Kane has improved his assistant skills quite well, which could be useful when playing in conjunction with Mbappe.
I believe you should flip that line; if Kylian Mbappe failed to come to Real Madrid, he would have missed out on his path to glory. The Frenchman will only discover what he is looking for when he joins the Spanish team, and that is the UEFA Champions League. He would fit in wonderfully in the team, and with his lightning speed, he will make valuable contributions to the club. Real Madrid have another player on their radar. Tottenham's star scorer, Harry Kane, is a top target for the Los Blancos, and I don't think they will get him easily. Tottenham Hotspur would like to put up a fight before allowing him to depart from the club.