Dev realised Ishwari will never let Dev and Sonakshi to be happy together and his conversation with Sinakshi made him clear now even Sonakshi has given up. He let her go, so that she can be happy and at peace.
He din leave Ishwari physically but took away the pride she had "mera Dev". He is no more the caring son and brother, he is no more the miney making machine for them, Ishwari used to boast about her son's character, he butchered his character.
He now knows that actually Sonakshi was toy for Ishwari whom she gave him because he was upset not having the toy. So, dear mom now that i don have the toy, u don have ur son.
This is the best way Dev could punish Ishwari.
I am happy, Dev had told his lawyer when he had come to get prenup signed from him " agar Sonakshi hi nahi hogi meri zindagi mein toh main iss paise, property aur buissness ka kya karoonga."
He actually left everything after she left.