Davies is already a better player now than he was at 19 but I guarantee he will be better by age 25.
You're also confused about how good players are when they "burst on the scene." Very few are actually top level and most of it is just a hype job.
In your opinion.
I see nothing to suggest he was as good as he was in his breakthrough season.
You can literally name tons of player who haven't improved - Macheda, Ravel Morrison, Walcott there are absolutely tons.
Which is why I judge a player with what I see with my own eyes - if Kean is strolling around the pitch like a drunk fan im not gonna overlook because I hope in 2-3 years time he may be better - even if he is better how do you quantify that? Better as in world class better or an extra goal a season better.
Feel free to judge a player how you do but lets not pretend its a proven formula as it really isn't.
You hope whereas I go with what I see.