Slave revolts were a thing in early history too though
Most slave revolts don't start in the harem though.
Revolts tend to occur when the tangible hazards and suffering of slave life start to outweigh the risks and consequences of fighting back. Slaves living in more or less pampered luxury are far less likely to rebel than ones sleeping on the ground, wearing rags, eating scraps, and getting whipped every day. Palace slaves and servants who are potentially living more fulfilling lives than impoverished freemen are far less likely to light that fuse.
Even in American slavery, there were distinctions between house slaves and field slaves - and the ones toiling in the fields, sweating and engaging in back-breaking work in the fields were far more likely to run away or fight back than the ones dressed up in finery, sleeping in comfortable beds, eating much better food, and who are serving their masters directly. Sure, a house slave could have a cruel master be more inclined to rebel, but loyalty to their masters (or at least the illusion of it due to self-interest) was just as common (if not more so). This is actually part of what caused divisions in "solidarity" between the slaves who were receiving far better treatment and the ones living in worse conditions - the field slaves would resent the house slaves who had "lucked out" and who were treated far better, while the house slaves looked down on their less fortunate kin, and would fear ever losing their own place and having to become like them (and could often be cruel to the other slaves themselves out of a sense of superiority, or would actively sell out attempts at resistance or rebellion to preserve their own preferential treatment). House slaves had far more to lose and much less to gain when talk of revolution came. Even if both types had some sense of kinship for each other, they were also divided by their own circumstance and position. And of course, the masters themselves would encourage that sort of separation, because it made revolts more difficult.
The idea that slaves would rebel over the nebulous idea of "freedom" alone (or the even more ridiculous idea that people should be "equal", or that "the people" should rule instead of one powerful overlord) is a very, very modern idea. For most of human history, slavery was accepted in one form or another because it was both useful and easy (and arguably an extention of the natural divide between the wealthy elites and the common laborer that already existed as an outgrowth of the Agricultural Revolution and the development of property, wealth, and power). It's really the growth of technology and the slow interconnectedness of the modern world that has sort of led to slavery being seen as both unnecessary and unacceptable.
Admittedly, this is WAAAAY overthinking the background setting details of a story that is ultimately about vests and bread. But it can be interesting to think about.
IIRC Cyrus did mention the sultan softening up when he got old, we don't know what he was like in the past, but if Cyrus saw something admirable, it probably wasn't all that great.
True - and it's almost a given that any given powerful historical leader has to remain cruel to keep their power, or at least need to be cruel to establish it in the first place. A ruler who is kind from the start will be seen as weak, losing their grip on power and encouraging their enemies to act. But a strong ruler with a reputation for brutal retaliation can eventually
afford to become more kindness or understanding, because then it looks more like mercy from someone strong enough to offer it rather than weakness. It is silk hiding steel - and the intelligent will always be wary of the moment when the facade of civility might drop and reveal the vindictive side.
That being said, from the point of view of a younger person who grew up with only a passing awareness of history, who has never seen that ruler as anything
other than kind or merciful, they're much less likely to see them as just another cruel dictator. Which is a door that swings two ways - a lovable street rat who only encounters the ruler in their more enlightened and mellow old age might see them more as a kindly old man worthy of respect, the ruler's strong, impetuous children and more likely to see them as a weak old fool who needs to be replaced...
But this also shades into the deeper complexities of rebelling against power in a non-modern setting. Someone whose family or loved ones suffered under the earlier, more brutal reign of a ruler will carry that resentment forward, and be far more likely to want to tear them and their legacy down even after they've matured and mellowed and become more kind and merciful.
A MC who has never suffered
directly at the hands of the Sultan (and who is actively benefitting from his position now) is far less likely to be willing to rebel than one who is nursing resentment for specific reasons (rather than just hating the system as a whole or resenting their own unfair origins). The higher you manage to climb the ladder, the more afraid you become of falling back down.