I completely get where you are coming from. I was the partner who didn't want children at all. I didn't want the responsibility, the loss of freedom and the financial burden.
We then had a child and any doubts I could have had dissipated: I now knew with absolute certainty that I did NOT want to raise children. No room for doubt whatsoever. I struggled with anger, depression and grief of my previous lifestyle.
A few years later I'm perfectly satisfied with not one but two kids. We would have a third if we were young enough.
All of this is to say: it is a life-changing experience and no amount of doubt or certainty before the fact can prepare anybody for the reality of it. Thus, people overthink it in my opinion.
Do or do not have kids, but let's not pretend that any of us knows what they are getting into until they are in the middle of it. Your priorities in life and your very own identity changes.
But that is not what I'm saying, is it? I'm saying that having doubts before having kids is very common, and that they dissipate once you actually become a parent.
Having children is an adventure for which you can never be prepared. Every parent takes a leap of faith and does the best they can, regardless of how much or how little confidence they had. That's it.
I would be more concerned about the ability of somebody to raise a child if they didn't feel apprehensive, because that would suggest they underestimate how much effort it is going to take.