Don't get my point confused- Son is a superior winger-striker "forward" in terms of 1 v 1 skills and finishing. But hes not a playmaker, and often fails to retain or advance possession when on the ball when facing packed, deep defenses. He seems to struggle a bit when he doesn't have space to employ his formidable pace.I'd still argue that Son is our most important attacker by way of him just being on the pitch.
Just take a close look at our games and see how focused opposing defenses are on stopping Son. Even when he's not directly contributing with an goal / assist, he's often drawing a couple defenders out and opening up space for others.
For instance, against SH - he provided an assist to Dier yes but the other 3 goals we scored, he drew double teams and opened up tons of space for the the attack.
Against Chelsea - it was his pressure and interception in the box that led to the first goal from PEH.
Against Wolverhampton - his corner that led to the goal.
It's no coincidence that when Son is off it, our team does not look anywhere near the same.
Kulu is excellent and one of the best young attackers in the league, but you can't say that he's more important to our attack than a player like Son who is the primary focus of defenses. Until he commands that kind of focus and attention from opposing defenses, it's difficult to compare him with the likes of Kane/Son.
Kulu on the other hand for me is a player "for all seasons". He's shown that he is deadly on the break, though not perhaps reaching "escape velocity" as often as Son. However, he's also good in packed boxes, is strong enough to hold off a CB, and can spot minute finesse passes that typically elude Son. He's a safe start for me against any side of any level.
TLDR Point being I trust Kulu to figure out how to break down any type of defense more than I would Son- who has struggled against deep-sitting bus parkers for much of his career. Of course if those fools would only play him fairly he would destroy them in open play, but they don't, and he hasn't quite figured out how to punish them for it with regularity.