And Rome itself remained unable to rebuild itself into a strong city because it's bishops somehow managed to reestablish something like the imperial Roman burea

And Rome itself remained unable to rebuild itself into a strong city because it's bishops somehow managed to reestablish something like the imperial Roman bureaucratic control on the religious side, establishing Catholicism as a centralistic "church empire" reaching farther out than the secular empire ever did (well, secular except for the occasional deification..). This influence of the city's bishop (aka pope, still elected in an utterly informal process by the citizenry for much of the middle ages) reliably prevented the development of stable power structures in the city, because papal succession was so important outside that whenever an imbalance between roman "families of power" was developing the weaker sides were immediately supported by outsiders (the stronger side would not be open to outside influence). Outsiders like the seemingly endless succession of German kings that went to Rome with an army hoping to get emperor status in exchange for helping the next pope into his job, but also by many others (e.g. Normans in what is now southern Italy). Imagine what the District of Columbia would be like if elections routinely involved violence, war between US states wasn't unheard of and (kind of like the inverse of the US Senate) nobody outside of DC had a vote for the presidency.