It's not a conclusion, it was a question. The Mana and Decadence systems are mechanics within these two similar games that are both controversial. Fans of these games seem to either love them or hate in equal measure. Seeing as how they're both polarizing features, I wanted to know more about them, which is why I asked the question.
And thanks for helping to inform me about said question. It sounds to me like the Decadence system is more in line with something like the Stability rating in Europa Universalis.
Ah, sorry. I might have appeared a bit impudent here. Honestly this mechanic is one of the best I have seen in strategy games to fix their pacing (usual early expansion struggle, mid-game challenge and late game boring domination). Stability is a decent analogy, but still not quite - since You have barely any way of affecting stability, it's growth and declne. Even in older good PDX games it was still a waiting game depending on how much money you invested in raising its level. Here it is extremely dynamic. You can always destroy buildings that affect its growth. You have decisions to reform the country to lower decadence. You can focus on just conquering the objectives and getting those regions don't raise Your negative nation size modifier. You can produce more culture to counteract its growth or invest in buildings that lower it. Also decadence grows on province by province basis, and the more You make in one, the faster it diminishes in time - so having one Babylon Orgypolis to make use of those powerful buildings and the Prude Countryside is actually a very viable strategy.
The only thing that You can't affect all that much, that affects decadence, is your government age, and even that You can try to fix a little with some decisions and some countries get special rules (Macedonia, Ptolemies and Antigonids grow old twice as fast unless as Macedonia You conquer a province a turn, and as Ptolemies Your Navy is at least half the size of Your land forces, then You don't age at all, and as Antigonids Your aging is related to how many wars You have - the more the better).
I am giving all those examples to show that this system is actually not controversial at all, nothing like mana, it is well thought out, deep and interactive.
THE ONLY flaw with it is that it does not play well with extremely simplistic diplomacy. It is a grand flaw, as playing Carthage the way it is meant to be played is nigh impossible, but they said they will be tweaking diplomacy in the first patch.
The biggest negative I see is the "you take it you own it" system, rather than the, actually great, implementation of occupation vs. ownership and peace negotiations, since it means that you can't defeat an enemy (as they usually take forever to agree to a cease fire) without taking his lands, which then leads to runaway decadence.
On one hand I agree, on the other, this system is more suited for modern times. While I agree we should have more tools for that in FoG:E, let's imagine in antique times, a monarch conquers a province and then says to his court "Actually... I didn't even want it, so we're getting out of here". There should be a way of abandoning provinces, or 'not conquering them' but it should come with disadvantages. The best thing I can think of would be a choice to pacify it and bring under your administration, or not, which would be faster but not extend supply to Your army all that well. Just so those punitive expeditions were possible