They aren't the only modern economies. Demographics plays a role.
>most of which operate immigration-friendly regimes
Recent policy and current demographics are two different things. These are still homogenous countries for the most part, especially Iceland which is exceedingly safe. Where demographics have shifted drastically away from the native population, the quality of life has reduced. (Look at Malmo, for example.)
>That happiness boost is nowhere to be seen when looking at real world examples.
http://archive.boston.com/news/globe/ideas/articles/2007/08/...
Here's a list of countries ranked by diversity: https://en.wikipedia.org/wiki/List_of_countries_ranked_by_et...
Doesn't look like a happy list.