People with multiple accounts would then have greater weight. With your proposed system, you just make 100 accounts, age them, and then become very trusted, and start scamming, or ruin/build up other's trust. That would also give people a free pass to scam newbies and jr.members because they couldn't voice their opinions. As it is now, the trust system is not moderated. Allowing staff to alter trust in any way would leave it completely useless. Staff members could then be influenced to do shady things for others or themselves.
As it is now, the people on the default trust list have a vested interest in the forum. First Theymos, those he picked, and from there those that the 2nd layer has picked, etc. If someone does something wrong with the trust system while on the default trust list, it effects the person above them, discouraging abuse, and encouraging those in a position to add others to the trustlist to think carefully about who they add, and act quickly to fix any situation that comes about. If you don't like the default trust system, individuals can remove the Default Trust list from their own account preferences, and replace it with people they personally trust.
I see your point. You actually touched another vital one - multiple accounts, which IMO shouldn't be allowed. People with multiple accounts are already using them to push their opinions, scam signature campaigns and so on. Not allowing newbies and juniors to leave trust ratings wouldn't be a problem, because it would discourage the most obvious spammers, those too lazy to even write a couple of posts.
You're right that it could be abused by people with multiple accounts, but allowing 1 rating per user should be fairly easy. It would only require the software to block multiple ratings by the same user. You should really consider this, even if you're not planning to change the default trust in any way.
As for the people on default trust, I'm not completely sure if people are responsible with their ratings. Just take Mr. CanaryInTheMine. He gave positive trust to a lot of people, who participated in his group buy, so in a way by buying something from him they also bought trust. The trust list is also rarely moderated. Mark Karpeles was on the list for months after his exchange imploded.
@malaimult I've seen some of these plants who bash certain mining companies while pushing their own through paid or unpaid signatures. A common practice these days
It's naive to think you can stop multiple accounts, you can't. I'm not going to sit here and list all the ways to do it, but suffice it to say the scammers know all of them, and it isn't very hard. It's their livelihood, it's what they do, and most of them are good at what they do. Multiple negative ratings by the same user already do not increase one's negative rating, but they shouldn't be stopped from giving more than one because there may be more than one issue at stake (there are good reasons to leave more than one negative feedback, and there are bad). Trust spam does get removed.
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.
If the trust list itself didn't exist and all comments were given equal weight, it would just turn into a numbers game, and scammers/spammers would win that game by a large margin. I found a spammer/account farmer in the local boards with over 130 accounts, and they weren't newbie accounts either. Under your system, that guy, if he wanted, would probably be the highest trusted member on the forums, lol. I've found other spammers with similar numbers, or even more.
Also newbies wouldn't know who to trust, and just throwing newbies in the water with the sharks when they don't know sharks exist (most forums ban anyone who's even suspected of scamming) isn't right. And if you're going to say ban scammers, we have no interest in banning people from participating in the forum just because they are breaking a law. Julian Assange, Edward Snowden, Guy Fawkes, fuck it even Robin Hood just to make the point, all "criminals" who break laws. (Alleged) rapist, (alleged) treasonist, (attempted) mass murder, and theft. All Criminals who should be banned if they posted here?