I'm not naive. You're paranoid. Any regime can be toppled by quick, decisive action and rapid improvements to pacify the discontents. If they have no reason to be angry, then they have no reason to keep fighting. It's that simple. It's not about hoping they'll make a moral decision, it's about giving two clear outcomes: fight us and die under your old leader, or live and reap the benefits under the new leader.
The Uchiha were planning a damn coup, they wanted to topple and control the government. That isn't the kind of thing you simply "pacify" you really think Hiruzen didn't already try to appease every single reasonable(key word here) demand? And your ultimatum at the end would do nothing but breed even more resentment and would serve to merely postpone the inevitable.
If the Uchiha clan turned their back Uchiha Madara, they'll shouldn't have had any problem turning their back on Uchiha fucking Fugaku.
You are completely ignoring the circumstances that led to Madara losing support. The Uchiha and Senju had finally achieved peace after years of brutal war and everyone was simply tired of it all and didn't want to fight. The Uchiha of "today" were the opposite, they realized Madara was right and were raring to take back control against their oppressors, Madara didn't have the support of the people, Fuguka absolutely did.
Fugaku would be completely discredited if his replacements improved conditions for the clan as a whole without shedding a single drop of blood. It would be undeniable proof that radical action was foolhardy and ill advised, and it would inspire confidence in the system.
You say this ambiguous "improve conditions" what are they? You think Danzo and the elders are ever gonna stop spying on them? No. You think they're gonna let an Uchiha ever be a Hokage? Hell no. You think they're gonna let the Uchiha just roam around the entire village right after they were planning a coup? No sir. You think the decades of resentment, mistrust, and grudges are just gonna magically disappear within the general populace? Absolutely not. There would be no proof as anything Konoha and Shisui try to do would be from a tainted well from the Uchiha's perspective and it would never be "enough", they already lost trust in the "system" a long time ago.
What resistance remained after the hardliners would be quelled at least for a time, during which, rather than let it fester, it could be dealt with.
That's a lot of hardliners considering the entire police force was likely in on it along with most of the adults who had a sharingan, they were all willing to die, what is Konoha gonna do with all these people? How are they gonna be dealt with in a way that won't be looked at as martial law? How can it not "fester"?
Because their "real leaders" were going to get them wiped out whereas the new one would do actual good and better their lives without risking it on something as ridiculous as a coup.
Their leaders were going to get them wiped out? Says who? Again you ignore the fact the Uchiha aren't gonna buy a goddamn thing Hiruzen says over their leaders. That old lady at the store wouldn't trust Hiruzen, she'd believe in her clan and not the people who've been spying on them with a gun ready to shoot.
Shisui would have to earn their trust by acting in their best interest, something he would have an easier time than the previous regime seeing as the Hokage would have then just gone through the effort of preventing a coup in the first place and wouldn't want to waste that progress by stonewalling the Uchiha again.
No, it doesn't. You are ill misguided if you think the entire Uchiha clan, including those that had ordinary lives outside out of MP, would rather rebel and die a pointless death than reap the benefits under new leadership. That's stupid on the face of it.