We do consider the planet
Well, you are not really, are you?
Are you even thinking about what may happen in the future, personally-wise. Job loss, sickness, divorce or bankruptcy. Anything can happen.
Can you mentally, emotionally, practically give your existing children what they need?
Reads like your egos are more important.
As a 1-child single parent, (he is now an adult), you really don't need a sibling for him. He will make his "family" around him. Mates, grandparents, cousins, uni friends, ex's-family abroad, etc.
Good luck with whatever you decide.