Anywhere that a system doesn't take care of itself or doesn't need any intervention is where decentralisation makes somewhat sense, bar a few exceptions. A place like a forum which is subject to tens of thousands of spam posts every week, probably isn't the best place to have a popularity contest for moderators or ones that have simply been here the longest. That's not how you get a system that keeps itself in check.

Decentralisation is brilliant, but only when correctly implemented into something that actually works better decentralised. For a currency that makes absolutely perfect sense. Potentially for holding certain records via the blockchain, that aren't data sensitive it makes perfect sense, but a forum it doesn't, and the moderator selection issue is only part of the problem. Very likely, users would've stopped using the forum before this issue even became apparent.