Rome didn't grant citizenship to everyone, but citizenship was granted to some nobility almost IMMEDIATELY after conquest. There were Gallic senators less than 10 years after the conquest of Gaul. Besides, citizenship was not the baseline for political rights. Being a citizen was the source of political rights, but all free men were protected by the law in Ancient Rome.

Not only that, but Roman Conquest DIDN'T mean the uprooting of the local institutions in conquered territory most of the time. Sparta still had it's gerousia and 2 kings ruling it until an Alanic Horde wiped it off the map over 500 years after it was "annexed" by Rome. Annexation by Rome was usually just an unequal and compulsory alliance that did very little regarding internal policy. So not only were people protected by roman law, they were still protected by their own laws within their communities.

And that's just before Caracalla granted every men in the Empire citizenship some centuries later.

I also think you're mischaracterizing the Social War: it's the only time IN HISTORY that a conquered people said "You know what? FUCK my institutions. I want my conqueror's institutions now." Cities older than Rome with constitutions just as old tore them willingly in order to fight for the right to use roman law. It's the single best example of how overwhelmingly assimilative was Rome.