"It is fallacy that there's a magic formula to what makes a pop star.
"If a member of the public was up here now their opinion would be as valid as hers."
Ayda also defended her credentials, saying: "I think for me, on a professional level, I've auditioned for years, I know what it's like to have the stakes high, to put it all out there.
"I know what it's like to get the job, I also know what it's like to lose the job.
"I'm excited to be that rock and stability for my group and help people achieve their dreams."
She added that her relationship with Robbie has given her plenty of experience that will prove useful.
"I think being with Rob for 12 years... I've been every step of the way with him. I have a huge amount of empathy for the process," she said.
"I know what it takes from creating the songs, to promoting, to live shows. It's a very emotional process."
She added that she was excited to be "the rock" but also the "underdog", adding she would "give the men a run for their money."
Cowell also backed Ayda, saying: "She's passionate about people and people doing well and has an optimism and I've tried to make an optimistic panel. I've never felt so excited."
Williams said the couple is undecided about whether their children will appear on the show, adding their daughter Teddy seems "destined" for a life in showbiz.
Robbie said, "She actually cries when I go on stage and she can't."