Just to illustrate how 'unsustainable' we are, I will use the actual example of my own family.
Let's start with my 4 grandparents about 100 years ago.
They have 10 children that we know about (probably a few more that we don't know, but let's stick to the ones we know).
Those 10 people had 21 children (my generation).
And we (those 21) had 45 children (so far!, because some of my cousins are still within reproductive age).
None of those 45 children has any offspring yet, but they are all about to start...
That give us a whopping 80 people, of which 74 of us are still walking on Earth. 9.25 x the original 8 in 100 years.
So, which is the 'unsustainable factor'?
For clarity, I am not suggesting 'supressing' people from having children, this is a personal and free choice... but we don't need to encourage them or make them feel that is 'their obligation' to have them because of us the oldies who are about to leave anyway... let them make their own choices.
(NB: I know there is a fallacy in the way I am counting here, because I am not counting the partners and the ancestors of all the partners in this calculation... but I hope you see my point).