Perhaps, for those who made it clear they won't return to the forum again, what is the essence of still leaving them in the DT? besides, the positive trust feedback from other DT members should be enough.
DT system is quite decentralized, even it is not perfect decentralized, so you can not remove any DT member and his trust feedback just because of his too long time of inactivity.
Your question is asked many times by other forum members and there will be no big change on it: remove inactive DT members and related trust feedbacks.
Individually, each forum member can exclude other members even DT members from his trust list.
theymos has his latest post on this issue.
it's still in consideration only.