Naina ji women are not weak among all species live on earth. You take example of tigress. She is not weak at all, she is the only one that gather food for the family! In case of human weakness seems to come from gene. I totally agree with you that at initial stage a mother had to close to her baby to take care of it and that's why the father needed to go for finding food. I think being occupied in hard work from initial stage, a men grew physically stronger and genetic behaviour take it to evolution in such way.
According to me, men are only physically strong but in other aspects they far behind of women. As example, women are comparably more matured at same age than men. And that's why to match maturity women generally choose older men that her. Your remark is correct situation will be reversed. Men will give birth to child and will be in the same situation as women is now in.
And in the end, a man may be physically stronger but they always bow in front of an woman(Not always: xd). Truth can't be denied.