I'm not an only, but my 2 of my 3 sisters and I never got along. Our aging mother's needs fall mostly upon me and my husband, and on top of that, the two sisters who like me least ask me for financial support.
I'm not "alone," though. I have an amazing husband, daugther, son-in-law and grandson, and a "chosen" family of well-matched friends for emotional support. This is also the pattern among the onlies I have known growing up.
So, please don't make a baby to be a companion or helper to your daughter. It's not fair to the new life, who should be welcomed into the family for it's own sake. In my observation, only maybe half of siblings like each other, and an older child often feels obliged to sacrifice for the baby.
My daughter was an only, and she mostly loved the experience. Now she has chosen to stop with her happy and well-socialized little boy, going on 6, who is quite clear about wanting to remain an only child. His life is well-balanced between play dates, school, and terrific one-on-one time with parents and extended family.