As I said, it's a gamble. If he turns out to become of Mbappe, Rooney or Ronaldo caliber, it will be worth it, but how often have we heard that someone is the next Ronaldo or next Messi? There is a lot of hype usually with young players who have great breakout season - in his case the hype is even greater because he is English. We even have our own hype with Januzaj who had a great first season. Where is he now?
My problem with gambling 100M on him is that in the attempt to fill out the hole at RW with Sancho we will be left with barely any budget for the rest of the gaping holes in our squad. Let's say we have 150M budget for next summer - I would rather get 3 solid players for RB, CB and CM and develop our own RW prospect (Chong), than blowing most of the budget on a prospect that we would need to develop and practically closing the door to Chong.