Too early to say whether Stones will become a top defender.
Early enough to say that he isn't one now though. So anyone who buys him is taking a gamble. Moreso now Everton are finacially better off, he will cost stupid money.
There must surely be other optons around the world, we could aquire for a more resonable outlay short term, plus our own academy products longer term.
We can't sign every young player who 'might' turn out a top one & we can't fill the team with 'maybes'. I don't see how Stones has put himself above all other young players in all positions, with his performances. He is just one option amongst many, some of whom will come through, some of whom won't.