The likes of Garnacho, Shaw, Martinez, Antony, Rashford, Sancho, Bruno could easily be on a title challenging team. But the most important thing is that you are looking too much forward 2 or 3 years is a long time in Football, in order to be a challenging team in 2 or 3 years we need to be good now, we need to improve every summer and you do that with a mix of players that fit your need, you don't do that exclusively with potential.
The question that people need to ask themselves when they focus too much on age, unless the player is at the required level today what makes you believe that he will reach that level in 2 or 3 years? That's why you need to think about today and tomorrow, you need players that are good today because with the right players you could challenge today and you also need to think about tomorrow. Both timelines should work in conjunction.
In the case of Kane from an age and level he would near perfect for us, he is ready made and we could plan with him for the next 3-4 years as a starter and bring a younger backup/replacement within that timeframe. Now the reason I have argued that I would prefer a younger and cheaper player is simply because without a striker this season we have managed to settled in the top 4-5, a good but not great player should see us be even better and spending less for that position this summer could maybe allow us to improve several other positions instead of focusing on the striker. But that's riskier than bringing someone as good as Kane.