The more I think about it, the more I feel that this trio was bound to fall apart because of the polar opposite personalities of Aastha and Shaan and the power of the Sabharwals.
Aastha is not the ones to take insults and she always raises her voice against injustice (barring the Raja case ). Sabharwals don't give an iota of respect to their bahus and that would be unacceptable to Aastha. Had she stayed back, their would be daily conflicts in the family because of Aastha's rebellious nature. Shaan would roll from one side to another like a "bin pendi ka lota". Shaurya would witness that every day and suffer from emotional trauma. I have seen how harmful the quarrels between parents can be for children. Living with the Sabharwals would eventually break Aastha's morale and she might have either lost her sanity or be pushed to the brink of suicide.
Had she fought for Shaurya, she had a 90 percent chance of losing, because winning the case would be an ego issue for the Sabharwals and they would leave no stone unturned to get a verdict in their favour. Both Aastha and Shaan would be subjected to vicious character assasination. Shaurya would be asked all sorts of questions in court and he would be asked to choose between his parents. No matter whose side the verdict went, Shaurya would still resent his parents for playing a tug of war with him at the centre.
If despite all this, Aastha managed to win and take Shaurya along, the Sabharwals would keep trying to get him back by giving him expensive gifts behind her back, or making him see how much he was missing out on because of not being with them. Not to mention the bullying that he would face in school or in neighborhood for being the child of a woman who separated from her husband. In our country, even now the woman is always blamed in the event of a divorce. Yes, he would get Aastha's upbringing but I doubt she would be able to give much of her time to him. A little Shaurya would always wonder why his mother could not stay with her father like everyone else's parents because that is the norm in our society. Maybe when he grew older he might understand but somewhere he would blame his mother still. And would hate his father for being spineless.
If both of them moved out, the Sabharwals would not let them live in peace and keep on creating problems for them. They let Aastha go because Shaan and Shaurya were still with them, but had all the three gone, they would not give in easily. Still this might be the best option for them and this is what Shaurya and Anokhi would probably do but their road is not going to be smooth, courtesy the Sabharwals.