People thought about Kawai, Butler and George too but they added pieces to their offensive games and it improved dramatically over the first three seasons.
Winslow has top tier defensive skills, basketball IQ, a willingness to play in the offense, and veteran poise.
If Winslow becomes Deng-ish on offense, you got a championship piece of elite defensive ability that can guard any perimeter player (And even some of the smaller 4s), run the break, makes great decisions with and without the ball, and has a high motor.
His floor is lockdown defender and high motor offensive ability that knows his role. But the Miami Heat are absolutely great at developing players (Lebron's post game, Chris Bosh's shooting, Wade's off-ball ability, etc) and Winslow is a gym rat but he has all the tools to be a good offensive player.