I think it's been long enough to say it definitively: Citadels are boring AF. They are crushing content creation. They are non-interactive and, especially in wormholes, discourage content.
Their effect on content creation is one of their "features", being that the defender gets a window of vulnerability in which it can be assaulted. The effect isn't that Citadels cannot and are not being destroyed, but that the effort of destruction isn't even going ahead. This is because the old dynamic of the 40 hour reinforcement is gone, replaced by predictable defender-friendly timers.
That's not all bad, I guess. EVE is a fairly antisocial game and not family friendly (ask my wife) and Citadels rebase the risk to times of the day and week that the defender can more usually be present to defend their shit, and not leave it vulnerable to off-hours attacks which can only be defended against via designing your POS to be boring, unfeasible, or too deadly for casual attack.
But this means that timers can't be gamed. You also have to face 3 of them, so the whole process drags out for such a inconveniently long period of time that it's essentially boredom defence in a way that can't be gamed (via Trashcats, diplomacy, or brute strength)
The DPS cap, meant to protect the weaker Astrahus and Fortizar from capital blobbing in lowsec and nullsec, basically means it's 30 minutes, flat, to achieve each stage. So there's a limited incentive for more people to get involved than is necessary.
The microscopic vulnerability windows, especially in w-space, also drastically reduces the opportunities for "fuck it why not" content creation. No more LOLfit POSs getting RFed and nubs learned.
Indeed, a properly installed Citadel which isn't coathangered on anchoring, is to all intensive porpoises invulnerable on a years-long timeline.
We still have POSs for moon goo and some capital build-outs. That's about the only alarm clock ops on offer, and the churn rate of moon goo POSs is very low. This is even with the lack of other opportunities for hotdrop o'clock content from Citadels anchoring. The alliances and coalitions which own the goo fountains maintain them as part of sov, or entrenched "ownership" of pockets of lowsec.
It will be a sad day when the moon goo POSs get swapped over to Citadels or Mining Platforms. The reality is that the only fights occurring on goo POSs are structures to take advantage of a percieved weakness in the TZ manpower of the owner, their logistical capacities eg; ability to bridge, cyno, reposition or wormhole back to the asset when it is attacked, and their batphone / blues list or reputation. Few get attacked by naive corporations just wandering in to lowsec and RFing a Cadmium moon for shits and giggles under Shadow Cartel's noses, and this gambit succeeding.
Citadelization of goo POSs will strangle the only realistic fifth column tactic available to a smaller organisation versus some of the absentee landlords, which is an alarm clock op that sees the attacker gain a benefit over the defender. If the timers and vulnerabilities of Mining Arrays are equivalent to Citadels, this will see these assets attackable only by people able to go head to head with the owners.
I'm sure the owners will sockpuppet themselves on the EVE-O feedback thread claiming that vulnerable goo POSs equals economic death and blah blah blah, but the reality is that no structure - POS or a vulnerable Citadel - can withstand a decent cap fleet, so why do Citadels need to be locked down for the vast majority of the TZ's and hours of the day? It's got nothing to do with the maths of siege; it's entirely to do with firewalling your shit behind artificial timer lockouts to prevent annoyance for you, and deny content to others.
This is something CCP needs to be wary of. And CCP needs to also add more danger to wormhole Citadels. Like, seriously, 8 hours a week?