...Default trust isn't perfect and incorruptible, but a trust list run by someone else (and let's be real here, if default trust didn't exist, someone would make a "default" that everyone would end up using anyway) would be much more corruptible...
This is quite an assumption to make. The forum itself is earning income and interacting with users of the forum. The moderators are paid, and that income comes from ads sold. There is a DIRECT FINANCIAL INTEREST in keeping this trust list under control of the people who are the primary beneficiaries of this (mods, any paid staff).
Even assuming that you are all 100% honest at your word, that alone is enough to influence your actions drastically regarding how you moderate the default trust. This is why a distributed solution to this is the only solution. Will it ever be exploited? Yes probably, but so is the current system. At least a distributed system has the ability to react and shift reputation to individuals who deserve it and remove it from those who don't THEMSELVES, not from a central position of a small group of otherwise disinterested financial beneficiaries.