I don't know mate, I think you might be underestimating Kovacic's defensive abilities. If we was playing 4-2-3-1, I'd be more than happy to play Jorgy-Kova pivot for example. There is still so much to see from him and I feel that to his standards his performances have been mediocre thus far. If he hits that top gear he'll be unstoppable, the complete player excelling both offensively and defensively.
And yeah, I agree with you last point. If Kante ends up starting against teams like West Ham from now on Sarri has to make sure he's not the furthest one forward.