Renewing Castle for season nine without Katic could open up the series to focus more on a case-of-the-week format rather than the serialized romance between Castle and Beckett. That would mark a change from what was envisioned for the series at the start of season eight, when new showrunners Alexi Hawley and Terence Paul Winter eyed a greater character-driven show rather than a straight-up procedural.
I haven't watched "Castle" since the middle of Season 5, but I can already tell you this is a stupid idea. The reasons I was so into "Castle" that I binge-watched 6 episodes a night before I finally caught up with the end of Season 4 was becasue:
1. Castle & Beckett.
2. Series-long storyline
3. The occasional 2-parter episodes every season
If they're going in a direction where they focus on more of "Case of the Week", they might as well cancel the show.