I would be very careful about making any promises to your wife. Dementia is a long, hard journey and you do not know what you might be promising.
At the beginning, mum wanted me to promise that she would never go into a home, but, instead, I promised that I would do my very best to do whatever would be the very best for her. She is now, in fact, in a care home - but it is the very best place for her to be. She could not cope with living at home and neither she nor I could cope with her living with my disabled husband and I. She is now content and well looked after, but if I had promised her that she would never go there she might still be neglecting herself, or, more likely, she would be in a CH anyway and I would be feeling really guilty.