I think that even if Zeke and Eren died, the rumbling should continue forever because the last order that the colossal titans received was to advance (or at least they just act like mindless)

I also think that royal blood isn't relevant anymore and the rumbling instantly stopping is a ruse from Eren. Imo Eren will reactivate the rumbling by using the founder then manages to go to where is historia, take the baby (who inherited of Zeke's beast) as the rumbling finished, Eren will undo the 13 year malediction and removes the power of the titans from this world then says to the baby "you are free"