Mbappe is Real Madrid's main target after the departure of Benzema, while Harry Kane is only an alternative if they fail to bring Mbappe from PSG. The situation experienced by these two players with their clubs is almost the same, they both have one more year left on their contracts and both are not willing to extend the contract offered.
If measured by the remaining one year of contract and the attitude of players who are reluctant to extend a new contract, the club should be humble by lowering their selling price when Real Madrid is willing to sign him rather than having to let them leave when their contracts expire. The transfer process for Mbappe and Harry Kane is very complicated, Real Madrid needs to take another path while waiting for his contract to expire in order to recruit him for free.
You are right. Harry Kane is just another option, but Real Madrid is serious to sign him if Tottenham sets a proper price. Unfortunately, Tottenham responds the interest of Real Madrid with an outrageous price for Harry Kane. Recently, I only heard the rumor Bayern wants to sign him, Real Madrid seems no longer negotiating with Tottenham.
Sure, Mbappe price should be lower than previous price because his contract is almost over. Mbappe also doesn't want to stay in PSG, so PSG management must respect him by setting a proper price for him. However, since Mbappe is a superstar, it is always complicated to sign him. Real Madrid must do more big efforts to ensure PSG management to let him go.