Tactics can evolve over time. The current premier league champions don't play with a number 9. If you look at the heat maps and touches, Firmino plays deeper than a traditional number 9 and Salah/Mane come into a narrower position between fullbacks/CBs and both have numbers up there in terms of goals.
All this isn't to say Kane is a bad player, I just think when you have Rashford, Sterling, Sancho all doing well at big clubs (Man Utd, Man City and Dortmund) who all play in more mobile front 3 attacks, then you might as well try and utilise them the same.