I'm very sorry for your loss and I agree with NavyandWhite that in the immediate aftermath of a death people need to do what they can for the grieving widow etc.
But honestly, this is not sustainable long term and I would be a bit concerned about your dh saying "this is just the way it is now". in a week or so, I'd want to know how is this going to pan out in everyday life.
It wouldn't be fair to his wife and daughter to spend every evening with his mother. equally his mother will eventually need to live her life, connect with friends, get used to being alone sometimes. Your dh can't become her substitute husband.
I have seen it with friends dealing with elderly parents that it can become all consuming. You can support, spend time, include the widow/widower in family events, have them over for dinner etc. But ultimately you also have to focus on the needs of your wife and children too.
When my dad died, my mother had to face being alone for the first time in more than 50 years. She did great in that she had friends and longstanding arrangements for coffee/lunch/etc. (made me realise how important it is to do this as you get older). But she also relied on my sister more than was fair. On the day of my dad's funeral my BIL said to my mum "don't worry, sister (his wife) will be up to see you every day" and I remember thinking that was going to be a huge burden and expectation - and it was. My sister had never visited them every day before but she did as soon as dad died and then it became an expectation. Honestly, it was in the years that followed, to the detriment of my sister's children.
So I am saying neither of you are wrong. It is ok to spend a lot of time with someone after a death. It is not ok for this to become the norm - it won't help your MIL get her own life in place and it will be bad for your daughter and yourself. Maybe see how things go and then sit down and chat.
Also, one thing that my sis and I both noticed was when my dad died, there was absolutely no room for our grief - my mother's grief was so all consuming for her. I didn't blame her but your dh is probably feeling a lot of grief and possibly is spending his time comforting his mother rather than expressing it. You could probably help him a lot with this.
again, sorry for your loss - and I mean yours. I loved my FIL and mourned him sincerely when he died.