I think it's perfectly reasonable for his DD not to be happy and for him to know that.
That doesn't mean that I necessarily think you should terminate. Your wants right now and his DD's wants might be different. It doesn't mean either of your feelings are invalid.
Ultimately, it sounds like this is your last chance to have a baby. He might leave you. He might physically be unable to keep up as the child gets older. He might die. In my family, we tend to die in our 70s and your partner is 60 already. What is normal in his family? I see at least one poster has brought out the 'we all live until our 90s' comment but that's not the norm for everyone. No one in my family has lived that long, on either side.
Do you want a baby more than a relationship? How would you feel about being a single mother, whether now or later in the child's life?
It doesn't sound like you're close to his DD at all, so when he dies (and statistically, he will die first), she's not going to play the comforting daughter and you're going to be on your own. How do you feel about that? Will you wish you had your own child to comfort you and share your grief?
Yes, you have to consider your partner's feelings. And he, if not you as well, needs to consider his DD's feelings. But you are the one with the final say.
You can't guarantee you will carry this child to term. But you can say that you want to try, and that's your choice. Personally... it sounds to me like, despite all the challenges and opinions around you, you want to try. No woman who wants to continue with her pregnancy should be pressured into ending it.
OP, make the right choice for you. Whichever choice that is. YOLO!