So, can you suggest alternatives to current trust system? Ok, even if red trust from inactive members would be removed, your account still would be full of negative feedbacks. You want to say that all these feedbacks aren't valid? OK, I'm not going deep into it and argue about it.
I only can see problem in some cases with feedbacks from inactive DT members, but I don't think that your case is one of these.
Removal of the negative feedbacks from inactive members will defeat the purpose of that system, the feedbacks serves as a warning and caution for other people that might engage the person that has the feedback in the future so the system works just fine. Of course OP would say that it's unfair because he got almost all of the DT members on his bad side not to mention that the feedback is mentioning extortion which I think is a pretty serious offense, I mean isn't that a crime irl? His case isn't definitely one of those unfair ones, DT members in my experience in the forum aren't vindictive and logical when it comes to giving someone the ever infamous red trust.