I understand him wanting change and cannot blame him, but I really want him to stay anyway :-). He is such a supercool guy.
But all hope is not lost. I remember that when I heard Michal Madej (lead designer on Witcher 1) left after finishing TW1, I was worried about how will TW2 turn out. And guess what, it turned out to be the best game since TW1 (and even better than TW1 I would say). So while I want Tomek back, I am sure CDP is full of great capable people that can go on delivering amazing experiences.
But yeah, it won't be the same.