Those who would give up essential liberty, to purchase a little temporary safety, deserve neither and will lose both.
Each of them thrived because they embraced diversity and freedom, etc. giving themselves access to a much larger pool of skilled talent.
Venice offers the clearest example. At its height, they could appoint someone as unconventional as a sub-Saharan Muslim to command their fleet (think of Othello). But the moment they shifted to a locals-only approach for key positions, their dominance began to crumble.
The problem is that usually locals feel cast aside. And while they too get the benefits, they rarely see them as such… They feel entitled and screwed. Don’t care about the big picture.
Ps. Of course this is very high level as each of these cities / states / etc collapsed under slightly different circumstances.