It's ahistorical to say the Panarin signing single handedly ended the rebuild.
What really, truly ended the rebuild was Fox waltzing onto the ice a Norris level defenseman, Tony DeAngelo becoming a 50 point D over night, Shesterkin busting onto the scene 10-2 and looking every bit the part of his pedigree, Lindgren also waltzing onto the ice a top 4 D, and a year of high flying offense from one of the best top 6s in hockey that year. Think of what an unbelievable injection of talent that was in addition to what looked like the best prospect pool in hockey. The mainboards was flooded with threads about how well the Rangers handled this.
I am not arguing they handled this well but hurdurrr Dolan shiny toyz is ridiculously over simplified. If an out-of-the-box norris and vezina winner fell into your lap, there's no f***ing way you'd have "stayed the course".