Whether Noblesse Oblige ever really existed is certainly a matter open for speculation. Without a doubt, hereditary noble families did exert some real political power in their spheres of influence, and much of it was probably benign. Ruling nobles were probably no more subject to whimsy as the average 20th century US president. They had teams of experts to counsel them (or delegate to) and legions of bureaucrats to make sure things went smoothly. A noble could very easily do very little and have her slack taken up by her support staff, while a truly capricious and selfish one would find her every evil desire mired in pages of red-tape and legislative inertia. The subsector Duke could be forgiven for thinking that they might be a little irrelevant to the day-to-day lives of their constituents, although such attitudes are routinely socialized away in the best private schools provided by wealth and rank.
For their part, the subjects of this situation probably felt little hostility and some genuine affection for the ruling elite. The occasional bread and circus, tabloid story, or marriage gala made for good distraction, and it probably didn't hurt that most of the nobles were pop stars and scientists, and not the landed gentry at all. The weight of imperial tax was probably hidden as a direct tax on each world's own government, which is a lot easier to find fault with than the glittering throne of the Emperor.
What was a more likely cause of the splintering of the Imperium are the feelings of nationalism and tribalism that ebb and flow through the decades of peace like a tide eroding a rocky coast. With no major external conflict, the denizens (and planetary leaders) of the Imperium were free to reflect on matters closer to home. Without something to unite them, the more important question became, "How am I different from the rest of my neighbors." Dulinor, not unlike Hitler, Reagan, and Robert Bruce, recognized these feelings and took advantage of public sentiment to insist upon his rights as an Illelishian (forget that these rights provided him with the opportunity to assassinate Strephon). He then used cultural rights as a platform in his campaign to legitimize his claim to the throne. But Dulinor was not alone in this: Norris seemed content to unite the Marches on common ground, the Vilani seceded on the basis of nostaglic racial pride, Brzk followed Norris' and the Vilani's examples. External forces sought new cause and opportunity to expand their own empires, and Aslan and Vargr family and pack units were availed of new opportunities to seek a greater independence.
It seems like the assassination was the event that made everyone realize that the Imperium wasn't inevitable. Once they realized that, it seemed that forceful resistance was possible, and ethnic difference proved to be all the reason most people needed to rise up and demand more direct control of their affairs.