If a GM comes into a loaded team who is ready to win, and can't get them over the hump in 3 years, then yea, maybe you fire him.
But what if a GM comes into a shitstorm with over payed, underachieving players, accompanied by a terrible farm system? Then, in less than 3 years he builds that into one of the top (or the top) systems in the league. No reason to fire him, at least I can't think of one. Especially now that he's cleaning house and the team will have a crapload of cap to fill out the roster in a couple years when the young guys take the next step.
I can't even judge him yet because we have no idea how all this will turn out, but most Kings fans wanted two things - the aging vets gone and an exciting young group of prospects. Blake is delivering on both right now. Not gonna lie, it's difficult at times to separate Blake the player from Blake the GM, but it's becoming easier as we see the direction the team is headed.