Williams will mark his farewell international appearance against Australia on December 3 after a record 57 tries for Wales including a remarkable strike rate in the RBS 6 Nations of 22 touchdowns in 41 appearances.
And the 34-year-old believes a lot of Wales' recent success comes down to the strength in unity of the coaching team.
"It's a great relief for us as players to know that Shaun's going to be staying. He's been very successful since he arrived," he said.
"I think the World Cup has certainly shown defensively we are up there with one of the best in the world.
"A lot of that is down to Shaun. I'm certainly very relieved that he's going to be there for a number of years and I'm sure the whole of Wales is.
"The coaching staff, the likes of Rob, Warren and Shaun, work very well with each other.
"They do work well off each other and that's great to have. It's pretty much like a rugby team really.
"You can have all the superstars you want, but if they don't work well with each other and don't get on with each other it's not going to work.
"I think that's why Rob, Gats and Shaun work well off each. They have worked with each other before and they're mates as well."