Do you think decadence is a legitimate cycle in history, and if so, does it belong as a mechanic and how? Not referring specifically to Islamic decadence.
Complacency/stagnation is the rhyme throughout history. I think the decadence mechanic in CK2 was the devs trying to go about implementing the Muslim trend of: Rise/consolidation -> decades/centuries of fuck-all -> decline/stagnation -> inexplicable collapse – if they weren't swallowed by a neighbour first.
Islam is a weird religion because it has within it one oxymoronic characteristic that's unusual to think about. It's a religion that incentivises war on non-adherents, whilst simultaneously disincentivising it. In Islam it's a sin to think about violence during peacetime, and it's also a sin to flee battle. The former stymies technological innovation, the latter means if you start a battle you know you're a risk of losing, then logically you ought not to declare it because you're doing something with full knowledge you're going to sin. It's like a Christian walking into a brothel knowing they'll jump into bed with the first harlot they see, or a Jew biting into a sandwich that it may contain pork – it's just not kosher.
Retreating = sin made Muslims risk-averse because starting a fight knowing you're at risk of being forced to flee means pissing off Allah, so it's less risky to avoid war altogether unless you already know in advance you're going to win it, or expect to, at least.*
So once a Muslim state reaches their ceiling, and the risk of waring with an equally sized neighbour is too high, they more or less just remain as-is until their stagnancy causes their decline. They can't innovate on how they commit/fight war because they're both lacking the necessity that drives innovation whilst at the same time can't do anything viewed as "unislamic" by their people or else said people are fully justified in removing them.
One thing that's unislamic is "violence" (not war, distinction applies here), so to innovate on new means of carrying out violence is unislamic, so trying to invent new shit to best their opponents at war in advance is a no-go - so is building up your army to be larger than your neighbours or something**.
There's a lot of weird little things like that which explain their civilisational trends and once you've become privy to this you can more or less see it everywhere. Since they also believe you can make up for sinful actions with virtuous ones,
You know how Communists say the Soviet Union wasn't real Communism? They think that because the USSR didn't do the whole "self-abolishment of the state"-thing that's supposed to happen under Marxism. This is effectively the same logic employed by Muslims regarding their "peaceful" religion.
TLDR: Islam hastens societal complacency since it shuns risk-taking and "violence" ergo no wars unless neighbour is certain to lose, no innovation in matters of "violence" (nothing says they can't task or commission a non-Muslim to do this on their behalf however or simply steal the innovations of non-Muslims), and keeping society virtuous to increase the odds of their people getting into heaven discourages rocking the boat too.
TLDRTLDR: Islam encourages stagnation since it lacks developmental inertia by discouraging anything Unislamic.
*
I think this in part lead to the Muslim use of slave soldiers (Janissaries/Mamluks) because in Islam a slave can't disobey their master, so being commanded to run as an obedient slave cancels out the sin caused by running from battle. The slaves themselves are a loophole because you can't enslave Muslims in Islam, but making them Muslim after the enslavement doesn't violate the rule. So effectively the use of slave soldiers are a double loophole in Islam to facilitate the creation of Muslim soldiers who can retreat without pissing off Allah. At the same time these guys are your elite fighting force so you shower them with shit to keep them happy, but when you've got nothing to send these guys against you've basically got a mass of highly trained, well-equipped soldiers who have all the justification in the world to throw a hissy fit whilst simultaneously not desiring the loss of their slave-status because it came with a lot of benefits.
**
The purpose of a Muslim ruler is to more or less shape society in such a way that guarantees their people can get to heaven, so even though some shit isn't strictly banned in Islam (Gambling or Drinking), since a "good Muslim" (paraphrasing) doesn't need either, it's better to force the common people to do without since it'll help them be more virtuous. Some instances of Islamic tolerance are partly because it doesn't matter if a Christian/Hindu/whoever doesn't follow Islam, because the priority is on how the Muslims comport themselves to best increase their odds of getting into heaven. This responsibility on the ruler is also a possible part of the "decadence" mechanic, because if Muslims see their ruler isn't doing a good job of making society virtuous, it then becomes a virtuous act in of itself to remove said ruler. This is also meant to be something of an anti-complacency mechanic within the Islamic faith itself because, like with he Mughals flip-flopping between tolerating and being intolerant of Hindus, it's probably helping to give the illusion of pushing society more towards virtue. Still doesn't help with stagnation though which inevitably kills these states. If a ruler sees things are "good enough" then they won't rock the boat, even as their neighbours surpass them. Surrendering in a war is also unislamic so one risky conflict, a loss of a significant chunk of the army, and a citizenry pissed off their leader who is now a sinner in the eyes of God, is one recipe for disaster. Casualties don't matter, so long as you "win". Saddam Hussein losing (in the process of losing) the war with the coalition had more to do with him being removed from power than the thousands who died in the war with Iran, it's kind've nuts.
(I think complacency and aversion to any and all risk can essentially explain a lot of the decline in the West. Anything see as a "risk" to the ruling classes is basically shunned, and everything is more or less a derivative of what's been done before or is designed to cause/prevent as much harm as possible.)