Nailing down the "regret not having" percent is difficult as the data often contains people who wanted to have children but were unable to (health, financial, etc. reasons).
Unfortunately not a reason for many who basically can't afford to have kids to still have them.

Dave his post here
https://www.eevblog.com/forum/chat/kids-and-happiness/msg6166147/#msg6166147 holds a big truth and I should have followed it, but here I am again. Yes there is no winner to be in this whole argument about kids and happiness. Only multiple groups with different opinions, that will never change their minds about the underlying subject, and that is ok.
To the future of society, sure a lot of people that have kids would want it to be a good one for those kids, but the question at the moment is if that will be the case, at least in the short term, and another question is, does it even matter in the long term.
Is the human race actually so great that it has to stay in existence for ever, especially looking at the greater scheme of the whole universe. What does it matter if we fade away and no evidence of our existence remains? (Who remembers the film "The meaning of life")
For this to be excepted we as a species have to step over our own ego for sure.
Our track record through the ages is not so great, and we do not seem to learn from it. Just look around you to see what is going on in the world at the moment.
On the subject of selfish, my offense taken is not about the selfish it self, it is about calling one side selfish, while it applies to both sides, and in fact it basically is a deep seated human trait. When it comes down to it we all are. Even doing good is practically always a selfish thing, why, because it makes
YOU feel good.
I know I'm selfish, egocentric, and a loner. Always have been, one could say it is in my genes, and I'm fine with that. Trying to change ones character is a hard thing to do, and not many will succeed in doing that. And that is ok, as long as you do not harm others with it.
Do you harm anyone by not having kids, maybe, maybe not. Same for having kids. Don't forget that some of them grow up to become a serial killer.

Yes I am the devils advocate.