Newbies already forced with Default Trust depth 2 if they don't change it, that's why its default. How couldn't it be better?
There are two main reasons why I think Default Trust is the better alternative. First of all, it is fairly transparent, and this provides for accountability. The trust lists of everyone at depth 1 are public, which has historically kept Default Trust mostly comprised of reputable members. This is not the case with the "suggested trust" list under the new system. People are added onto this list based on the trust lists of everyone over full member. These lists are entirely private and anonymous, unlike direct positive/negative feedback, which I think is dangerous. It would be fairly easy to manipulate the "suggested trust" list as an individual, and even easier as a group. One could essentially buy their way onto the "suggested trust" list by purchasing or creating a decent number of accounts and adding themselves to the accounts' trust lists/negatively trusting other accounts to lower their comparable trust values. People complain that Default Trust is an "old boys network", this will be even more true under the proposed system. It will be very easy for a group of high activity accounts to trust one another and solidify their places in the trust network without actually earning them. Activity has nothing to do with trading and should not be the basis for the underlying trust values accounts hold, which this system seems to suggest it should.
The other issue I see is how difficult it is to actually build one's own trust network from scratch. This isn't an issue to most people debating over the two systems, since we already have a good idea of who we do and do not trust on the forums, regardless of which system is used. This isn't true of a new user. Take a look at someone like Vod's trust without anyone on your trust list (everyone's feedback is untrusted and they are at +0/-0) and no knowledge of the users' histories who have left him trust. I would not add Vod to my personal trust list if I was a new user, since the majority of his feedback is negative, and I have no reason to believe that his positive feedback carries a lot of weight. Obviously, Vod is a very trustworthy user, but you wouldn't know that if you didn't have a history on the forums.
My point here is that Default Trust gives a new user a good starting point about who to trust and who not to trust, while this new system asks them to pick their own "Default Trust" pretty much at random, since they will probably have little reason to pick one name over another. After a few months here, most people have decided in their own minds who they trust and who they don't, whether they have bothered to edit their trust list or not. Forcing people to edit their trust list (which is what the proposed system essentially does) will hurt new users and change nothing for longtime users, so why make this change?