Maybe you should find better neighbors and get to a library where you can read population statistics.
I've read and watched many videos on matters of sociology on top of working with poor people most of my life. I can tell you with certainty statistics are void of many complexity's.. and only take into account a population that derived from inequality and booming yet unsustainable industries. This is the problem, trying to look at the world population through a numerical lens. Don't believe for a second the figures on immigration though, you could add 20% to that in most western country's with safety.
You should but likely won't rethink what you've read on this matter from another perspective. I did make the same conclusion you did a few year back, I fully agree 'native' populations are dropping in said country's.. but it won't drop much past a sustainable level. Like over filling a life raft in calm seas. Good luck in the storm.
The deeper complexity is that very few western country's have a dominant age old native blood line. We are all mixing in the same pot and have been for centuries, drop that mentality if you want to understand deeper. What your statistics fail to take into account is that first wave immigration to a steady economy will lead to some 3rd generation mixed race children who on passport are considered native to said country. A western country that has work will never drop it's population below the countless idiotic doomsday predictions. I'd expect it to do some crazy things over the next 20 years though. The end of the oil and transition into renewable will no doubt create more boom in w/e country's spear head it. Hopefully poor country's are able to get a stake in it otherwise things will continue much the same as it has with oil, minus the wars (a small consolation I guess).
The only thing that will lead to a genuine alarming drop in any western country population is by fully closing the borders prior to suffering a large loss of life via war, famine, etc.
I'm holding out that you can give me some of your own insight into your conclusions rather than just telling me to go read.