honestly i cant take simon word at face value, he is a manipulative and a good liar, one example are his contradiction about the doomed king plan that some hero will kill him by becoming the most hated person like lelouch, and ginasta fit that role but somehow he say to her doing that will make the society back into chaos and civil war despite the flaw that ginasta can just kill simon (not the doom king) and someone else can pretend to be the doom king (riala and robin probably will found a way) to maintain the status quo until it get overthrown, it dont help that he try to enrich the society making the populace accept the tyrant rule (literally sabotaging his own plan which i believe are just his bullshit to woo the girls to give the impression simon are a saint and accept to do his bidding) the harem literally mention it and simon decide to expand the doomed king plan and repeat the samething again. does ppl really believe someday the society or community unite together to overthrown the doom king?
The primary goal of the doomed king plan is to "enrich society" and make positive government changes, that was always the plan first and foremost. The part about the doom king becoming unpopular and being killed was the idea to give Simon an escape from being a dictator for the rest of time, but only after he finished improving the kingdom with his government reforms. If there was a clear opposition to the Doom King's rule then there would be no power vaccum when he was taken out and the new government could keep most of his positive changes while also distancing themselves from the tyrannical roots of the Doom King's rule.
What went wrong with the doomed king plan was that Simon underestimated just how popular a better government would be, even if the one in charge was a coarse and brute tyrant like the Doom King. He thought being a bossy dictator would make people resent the Doom King in spite of his decisions resulting in an objectively better government, and he tried to distance the Doom King from most of the good works (he tries to associate most of the good works to Janine and Sarai during chapter 3), but people in general are still eager to give the credit to the man in charge.
Also Ginasta would have been a prime candidate to "kill" the Doom King and "liberate" the opressed people under his rule, but she came too early, she attacks Simon at the start of chapter 3 when he had just barely finished stabilizing a post-invasion Yhilin and before he achieved any government changes. If Ginasta killed him then, no positive changes would have been enacted and there would be a power vaccuum that would throw Yhilin right back into chaos. Also Robin was not able to impersonate the Doom King back in chapter 3, and Riala had been lobotomized by the Incubus Emperor, so the plan could not move forward if Simon was to die (also why would Simon be okay with Ginasta killing him but not the "Doom King" lol, are we condemning Simon for having no desire to die?).
Idk what the dev has planned for the ending and the resolution of the doom king plan v.2, the Doom King in chapter 5 has been making or being blamed for several unpopular decisions, like destroying the Crystal of Ivala, and the governments of Tak'kan and the Chalice States have been given a great degree of autonomy, to the point where even if the Doom King isn't resented as a leader, he could be seem as "unnecessary", so the plan to have the Doom King be removed from power is now more viable than ever.
But at the same time, one of the main reasons why Simon didn't want to be a ruler forever was because all his lovers would eventually die of old age, he wanted to be able to leave his position as ruler of a kingdom to enjoy some peaceful time with his lovers before it was too late. Now that the angle is to make everyone in the harem immortal, there isn't much of a reason as to why the "Doom King", or Simon as a ruler in any capacity, needs to go away for good. Maybe the dev will make multiple endings where the player can choose if Simon removes himself as a ruler after the Doom King "dies", or if he takes over after "usurping" the Doom King.