so why did ayan make brahmastra as a love story

so brahmastra had a lot of potential in its story it was good in terms of graphics and songs but i dont understand why ayan included rabir and alia love story as the main plot point of the story . most likely cause ranbir and alia are married in real life . when big b started talking about dev i was like finally we have a great villain . only for big b to say dev is ranbir father . my mind kind of got saturated by that ending where their so called pure love defeats brahmastra are we living in the 90s or something . saving graces were SRK , nagarjuna , big b and the post credits scene where dev gets freed . so i am curious why such a intresting setting for a movie needed love story as its main plotline ..

https://redd.it/1ww167i
@bollybng