So if a scammer made it onto default trust (it is inevitable in a decentralized system rife with scammers), would excluding everyone that included that scammer remove the scammer from default trust?
If the scammer is in DT2: it seems to me that excluding the scammer directly is a more effective way of achieving that goal. Also easier to petition other DT1 members to do the same, which may be necessary - by definition a DT2 member needs at least two exclusions.
If the scammer is in DT1: excluding everyone who included the scammer won't change anything, you (and other DT1 members) have to exclude the scammer directly. Exclusions don't matter for DT1 "election". The users you exclude would still be able to cast their 10-merit and 250-merit votes and elect the scammer.