But having kids is all-in: there is no turning back and no flexibility
I would feel so much more limited and bounded in what I can do in the world, because my own financial future and my own mortality would be way more relevant with dependents.
Yeah, it's just biology. Works for some, not for others. The problem with the advice 'have kids for meaning in life' is if you do have kids and you get nothing out of it, you might resent the kid which would ruin two lives.
That said, I didn't have my first kid until my late 30s and every thing I did prior to that seems like a pointless waste of time now. YMMV.
Isn't that precisely why it works? Any life change you make where you have an out, you're likely to bail the first time it goes a little bit wrong. When I'm thinking about decisions like this, I remember one of the battles in Romance of the Three Kingdoms where the general (sorry, forgetting who) just deliberately leaves his army with their back to the water to make it clear, the only way is forward.
I would feel so much more limited and bounded in what I can do in the world, because my own financial future and my own mortality would be way more relevant with dependents.