Maybe I am misunderstanding you here but "zero risk" in category three is simply not remotely true. There is a very real risk that we could buy a young player and he just turns out to be not good enough and he essentially washes out with no value (or negligible). It happens all the time; it's not only not zero risk it is in fact MORE likely than him increasing in value. Are you seriously suggesting every 21-23 year old purchased is a success?
You're misunderstanding me.
Category three is essentially "during the contract with us will you get old enough that no-one else will buy you unless you're RIDICULOUSLY good". In Kone's case, that chance is very high... for someone in their early twenties the chance is as low as can be.
The situation you're describing fits into what I called category two: "during the contract, will your performance on the field raise YOUR value?" - I agreed with you in advance by conceding that this is more difficult to predict in younger players than it is in older ones.
However,
all other things being equal, a 33 year-old average player is worth very little whereas a 24 year-old average player will always be worth more.
So £5m on Kone is more of a risk than £5m on an up-and-coming 20/21 year-old, because if they both simply perform adequately over a three or four year stretch Kone has no resale value whereas the younger alternative has some at least.
It's all academic if he signs though; put on the royal blue and I'll support you until you prove you don't deserve it.