if they removed from the DT network, their feedbacks on those scammers will be invalidated since they aren't in the trust network anymore. it will be a mess and a lot of work for other DT members to retagged all of the users who were tagged by DT users who were gone in this forum months or years ago.
But conversely, it also means that the tags are permanent and even if a user proves themselves trustworthiness again, or their earlier accusation is dropped the tag would still remain as the DT members are no longer active. This also works in cases of users they positively trusted who prove not to be trustworthy.
Trustworthiness is not static and should be changing as circumstances do.