That 150M number seems about right.
That's not remotely the number that would move if they could.
If you walked into a household in a poor country and just offered them zero-friction citizenship to the US/UK/CA/Europe for their whole family, with the promise of a decent job etc. - no strings attached ... the number would be extremely high.
We're not supposed to use anecdotes here on HN but frankly it's life experience that has exposed me to the nuances of the most important things. Visiting these places and one kind of get an idea.
It's not colonialist, arrogant, condescending or anything. It's just a simple pragmatic reality of people's will to just have stability, safety and a bit of aspiration.
And yes, the answer mostly lies in helping these countries get much richer, and as I posted below, it's hard ...
But we are making progress. The number of 'absurdly poor' people, living on like $1 a day has dropped dramatically over the last few decades as the industrial revolution of 'making stuff' has started working it's gears in more traditionally poor places ... though that has come with a lot of baggage as well, like pollution etc..