It's not as easy as just saying "let's sign a striker that can make Kane work for his place" though. There aren't many good striker options out there, especially not ones who will feel they want to come to us and have to fight for a place. They type of player you are talking about is already first place at their club, and there is a very real chance they could struggle for game time by coming to us.
Realistically, there is maybe what, 20 strikers in the world right now who would guarantee a start ahead of Kane. Of those 20, 20 are already on shockingly high wages. As for players ready to come in and give him competition, I can't think of any. Is Saido Berahino the player to give Kane competition? I doubt it. He can't even give Solomon Rondon adequate competition.
If you can name us a player who can give Kane competition, won't demand extravagant wages, whose club won't mind losing him and who won't be a little primadonna if they can't get a game over Kane then that is who we should go for. But that target doesn't exist, which is why Totti's point about us needing to go for someone, who just like N'Jie, isn't quite there yet is our only possible option. It's that or we go for some who's currently not terrible, but will never be half the player Kane is.