No not wishful thinking nor theory.

Read about it here.

The decision about Boone’s exit was made early on, well ahead of The Blacklist‘s Season 9 renewal in January. That allowed the writers to craft the arc for her character, Elizabeth “Liz”” Keen, this season as her final chapter. I hear the decision was mutual, and Boone already is plotting her next move.

Well...I was one of the people who said The Blacklist can survive without Liz, that Spader is the draw, not her. We're about to find out firsthand if that's true or not next season.