Good for you!
I think it is great that you have rekindled your joy for riding, even at the young age of 40. As long as you ride responsibly and don't try to ride/race over your head you will be okay.
I understand where your wife is coming from. When our youngest was 6 months old, my husband tore his ACL and fractured the bones above and below the knee. He was on crutches for 2-3 weeks. It was very hard on me, becuase not only was I the only one who could carry the 6 month old, but also be responsible for carrying all the groceries, mowing the lawn, and taking care of my hubby and other daughter. It was extremely difficult on our marriage.
Seeing the stress he caused, he almost quit riding all together. I didn't let him. Instead he moved down to a 125cc bike, promised to ride in the "over 30" class IF he raced, and vowed to always wear full gear and ride within his limits. Every once in a while he tries to jump BIG, but he swears he sees the pictures of his children's faces on the takeoffs of the jumps.
Just stress to your wife that you will ride responsibly, if she still doesn't give in, then wait until your newborn is a little older and the family life settles down. I play indoor soccer and I could get hurt just as easily, but I waited until both kids were old enough to help around the house before I signed up.
Good Luck,
Mrs. B:)