Could happen multiple ways. Unpopular opinion on Lemmy who like to bury their head in the sand.
Western civilation is going to die from immigration. Places like the UK the largest cities are less than 50% white British. It's worse the younger down the age groups you get.
There is also the issue of population growth. I'm not sure our best and brightest are having the most kids anywhere in the world really yet world population increases.
The trajectory we are on is that we are doomed or maybe a country like China will make some difficult decisions that western countries won't and that will benefit them long term.
Best case scenario is AI gets to the point it can save us from ourselves. Which is the scariest thing of all.
Within the next 100 years maybe 200. The west will continue to weaken from multiple directions. The third world will continue to increase population and overuse their resources and miss manage the economy. Global warming will throw petrol onto the fire and mass immigration will happen. Some big war is also going to kick off and that will be the collapse of society (but I don't think so severe a new one can't be built). That will be the end of the West being the greatest place to live.