I'm not saying it can't/won't happen, but I don't think there is a logical conclusion to draw that the absence of Kane will guarantee an increase in Son's output. It's not as though we get an allocation of goals that are to be shared around the team and Kane has been hogging them.
There's every chance that we will, for a while at least, look impotent if/when Kane leaves.
Also worth bearing in mind that Son mightn't be as reliable as Kane from the spot, and has always been a streaky player. I should imagine he will remain that even as the main man.