In its first seasons, the more serialized Rick and Morty was seemingly on the cancellation bubble regardless of its perceived success.
Keep in mind that Adult Swim’s renewal was for 70 episodes, which is a lot for a densely animated show like this.
The overall gist is that Rick and Morty may still hearken back to prior plots and characters, but the creative team is going to do what it wants without being so strictly guided by canon.
Perhaps the idea of tying every story element together was more plausible when Dan Harmon and Justin Roiland thought the output would be more limited, but the renewal gives the show a lot more ground to spread its absurdity-embracing wings.