This is the worst advice to give. He chose not to have with the one he is married to. And you are saying to turn his back on that same person. And have it with someone else!
Marriage was invented when there was no contraception and life expecancy was half of what it was now.. No one should be miserable for the sake of "being married".
Marriage is between adults and life expectancy for adults was nearly the same as it is now.
I would expect HN to know that life expectancy in earlier times was suppressed due to child mortality.
Invented is such a terrible word. It was a contract. And within that contract was a place for love to grow without the idea the person was going to leave. And within that love was a place for family.
Today some celebrate the inverse. Have family. Then love. Then marry. Then divorce. And leaves little to no safe place for any of the above.
In some case yes. "Hope for the best, plan for the worst as people say".
The OP regrets not having children. His younger self made that decision. He is not too old to go and have kids with someone else or adopt (say as a single dad even).
Go have kids man! If your wife doesn't want to then get divorced. If she does then work together. IVD, adoption etc...
You remain faithful to the contract, or you are violating the contract in bad faith.
There’s no requirement to give a reason or even have a reason to end a marriage.
Marriage is not some form of slavery. It's painful to have dissolved... But very doable if the situation is bad enough.
Now the problem with op, is that it appears he is depressed and burnt-out, and is transferring the depression to a biological drive for children. To do this would be unhealthy for everyone, including any children they bring in the world.