If the colonial countries hadn't used the colonies resources to become rich, there would probably not be such a great difference between the wealth of the (former)colonists and the wealth of those colonised, such that the immigrants would not be attracted to come to share in this wealth. However, this lack of colonisation may not have made them any richer, the resources having been wasted (or still dormant).
It therefore seems apparent that if the wealthier countries gave away their wealth to the poorer ('levelling up' I think is the phrase in vogue) then they would not be so attractive for immigrants. I suspect that the wealthier countries would rather swallow the immigration pill rather than do that.
When the (former) colonial wealthier countries are so dominant, it is difficult for the former colonies to build their economies (our own [former] membership of a wealthier countries club shutting out former colonies imports being an example).
What happened to the resource rich country that kicked the UK out and used it's own resources to build itself, United it States? Didn't it became the World's most powerful and who the former Colonial Power can't do jack without it's support militarily. Who knows India could've been an even greater super power if it'd had the 250 years head start The USA had.
And whose population do you cut ? The O.P has given an idea in his killing off the old thread who he wants to go in the UK.
You need a healthy birth rate to provide if your 40 you need enough being born now to be earning to look after you and pay for the services you need and want in 25 years time. If the birth rate doesn't keep up you need to supplant it with economically active immigrants if they are coming from the same areas as they are now then climate change is going to make their countries not
liveable.
China's one child policy looked to be addressing the population growth but without siblings or extended family you'll be asking one child to look after his parents and probably grandparents.
The longer lived wealthier western nations using the highest amount of resources per head need but won't for whatever reason contribute, while the high birth rate in poorer low resources using populations need/want work and somewhere to comfortable to live