The Thornton's house is on mill property, so giving up the mill also means losing their home. This is not even mentioned by Mrs. Thornton, however, when John tells her that he is giving up his lease. Her only concern is for her son, and the blow this is to him. Indeed, she is far less accepting of the loss than John is: railing against the unfairness of it after all his years of honest labour and hard work.
There is one other possible cause for optimism: if Mr. Thornton is to give up his lease, he must go to London and meet with his landlord- or rather, his landlady- as he is now, since the death of Mr. Bell, Margaret's tenant. It seems that their paths must cross again.