It was only discarded when the centre was ABLE to control, and even micromanage, the peripheries.

This was only true when a metropole was massively more powerful than the peripheries (European colonisation of Africa) and able to impose "us" over "them"), or the metropole was in almost instantaneous communications with the peripheries (making the distinction between metropole and peripheries irrelevant).

Otherwise, the peripheries are almost as powerful as the metropole AND isolated from the metropole.  How do you build a common community then?  You co-opt the local leaders to identifying with the "king on the other side of the hill", and aligning their interests with their neighbours and their lord - aka feudalism.

Yes, feudalism is not stable.  However, the alternative in those circumstances (small autonomous communities) are also unstable.  And a feudal state is able to prey on smaller autonomous communities.

I agree with Catherine's assessment on the metastability of cycling between "Imperium" and "Long Night".  Really, given the constraints of jump technology, it is hard to see an alternative!

[ps. I think Phil's conception of "modern nation state in space" would be worth chewing through as a model for the Solomani Confederation ...]