This basically. They didn’t want Sydney or Vaughn to be the “bad guys” in the love triangle. I thought the first half of the season when Lauren and Sydney we’re moving towards friendship (the car chase scene is one of the few scenes I still remember and remember liking) was great. No one could replace Frankie or even Will, but Sydney needed a friend her own age to round her out, and after season 2, we lost that part of her personality and the show.
Would have preferred if Sark and Allison had continued to be the “evil twins”. They could have recast Allison with her “original face” if Frankie’s actress wasn’t returning (though I feel she was fired which was stupid). Sydney had actual history with Allison, a vendetta that ran deep, and it would have made their rivalry more epic than what basically boiled down to Betty vs Veronica but with Spies!
The heart of the show was always Jack and Sydney. Vaughn was just Sydney’s love interest and they should have kept him that way, as the stereotypical Chick - he’s her handler, the analyst and the guy in the chair, and not Sydney’s partner in the field. They kept trying to make him edgy with his father’s backstory and with Lauren, then a new identity and it was like the writers didn’t understand why his s1/2 dynamic with Sydney worked in the first place.