How would you battle against the fraud of making multiple (few, dozens, hundreds) of multiple accounts? The forum would be quickly overrun with fraud since the honest people would have the one account and fighting against the scammer with many. So, no, the reputation system would not work just the same.
You would battle it by building your own trust list, one person at a time. A decentralized network wouldn't need to mean that all feedback is equally values, it would simply mean that each user chooses which feedback to value. If default trust were removed tomorrow, I imagine that a huge number of people would immediately add badbear, tomatocage, vod, dooglus, etc, etc to their trust lists. The difference between that and what we have today is that everyone who did that would suddently know who they were trusting and what their reasons were for doing so, rather than being handed a crutch to lean on which is probably going to fail them if they start leaning too hard.
I would imagine that in such a scenario, over time, several tightly knit trust sub-networks might develop, and who knows, people might start choosing which trust network to "buy" into based on individual motivations and perceptions about those networks. But the crucial thing is that these networks would develop organically based on actual experineces of the forum users, rather than being imposed by a central, overworked authority which is subject to manipulation.
A case in point for this kind of reasoning is that even bringing up such ideas will get you shouted at (often by people on default trust). I was told in no uncertain terms by someone on default trust list that arguing for a different sort of trust system is tantamount to promoting scams an fraud---and that's not the worst of it, but I wont' go on further on that now.
There's another important point here, you don't have to go whole-hog. You could start with some small changes which would encourage users to interact with and build their own trust lists. A simple first step would be to change the overblown "warning: trade with extreme caution" to "this user has received negative feedback from someone in your trust list". That kind of change would encourage users to try to figure out what their trust list is, and how they can use it.
You can also create your own trust lists and remove and add anyone you wish so I guess you can argue it is decentralised.
You're right, hilarious, in principle. But in implementation, it's a bit of a fail. Everyone is invited to create their own trust list and to set the depth they wish to see in that list. But in practice, almost no one does this, so, because trust depth default is 2, unless you're in the depth 1, it doesn't matter to anyone who you've put in your trust list, and changing your trust list just amounts to not seeing what everyone else sees.
dserrano5 said
here that default trust wasn't supposed to last, but was added as a bootstrap mechanism so that the trust network wasn't terribly empty and sparse before people had a chance to create their own lists. The current system is a far-cry from that scenario, I think there are reasonably and concrete ways to improve the system. I hope that they'll be more seriously considered given all the abuses we've seen of the trust system by an (admittedly small) number of folks associated with it.
EDIT: for reference, a similar topic was discused here some months previously:
https://bitcointalksearch.org/topic/trim-or-eliminate-default-trust-1031791