Arguing about kids vs. cats is not useful if you don't consider the many factors that should be considered when having kids, even if the focus of your argument is one of environmental impact. For example, say I choose to have kids purely because I thought that might someday save the environment, but ignore the fact that I'm not well equipped to raise children to begin with (a factor that lessens the chances of future success), my plan starts to sound like a pretty bad one.
An earlier comment pointed out that many younger people are deciding to have pets instead of kids. That comment did not explore why. Without understanding why (I added my why as an anecdote), any argument about the baseline merits of one vs. the other is not going to be an argument worth considering.
> Of course they are some bad apples but that's mostly due environment factors
Breaking out some personal information now: I was raised in an abusive home. I was deprived of a traditional upbringing/education, and have spent most of my adult life learning how to be an adult.
I've also found success, but it's been complicated.
One thing I know for sure: having kids will never be the right thing for me, and that's ok.
Everyone has a different situation, but your commentary does not leave room for the wide spectrum of circumstances that people are dealing with.
> but your kids might do it or their children so you could say children are a good long term investment
Or they might not. To have kids solely for this reason is extremely questionable.