Kane's been the most consistent performer on the team and his stats are nothing short of exceptional.
But the thing is, if you put Son in Kane's role, Son would also get 30+ goals a year. Since 2016, the guy has averaged around 20 goals and 8-10 assists a year in a relatively limited role. Had he been a full time/every game starter like Kane (he came off the bench around 30% of the time), took penalties instead of Kane (accounts for at least 5 goals a season), and been the main target up front as our #9, he would have gotten 30+ goals.
We saw what kind of results Son produced when he filled in for Kane as our main man up front last year - it was Son that led us to the final in Kane's absence. Now that Eriksen is all but gone, you're really starting to see Son step up as both a scorer and playmaker to fill in some of the void that Ericksen left behind. His ability to succeed in multiple roles/positions is nothing short of amazing.
Kane, while one of the top strikers in the world, does not have the raw talent/natural ability that Son has - he's reached his physical ceiling (some might say it has deteriorated a little) and plays one position but does it exceptionally well. What Kane is at the moment is that he is Spurs' undisputed leader and much more refined and thus more consistent and clinical than Son.
Son is not as refined but he's still got a lot of raw untapped talent. You're never going to see Kane, or anyone for that matter other than maybe Messi and a prime Ronaldo scoring the goal he scored a couple days ago. Son can pretty much do everything Kane can except Kane's aerial ability but the opposite isn't true. Scary thing is, Son's getting better and better. He hasn't reached his ceilingi yet.