Why is Anika staying married? Because...

The reason is : Dadi

Anika feels endebted to dadi.
Dadi helped her and gave her this job.
She takes care of her.
She treats her nicely.
She got her the huge loan for sahil.

All in all, Anika considers dadi her family. And Anika is damn protective of her family so she'll go to any length to keep them happy.

I think Anika will stay in this marriage forever because of this gratitude she has towards dadi.
Partly because Anika feels dadi has done so much for her and sahil and no amount of pain or repayment can be enough.

I don't think it's cus of shivaay. Cus they showed them "falling in love" but they never actually fell "in love."

So 100% she'll stay in this marriage for dadi's sake until dadi sets her "free."