Lets say you are trying to buy 2 BTC for USD and the person you deal with ends up scamming you, he takes your money and he is gone (yes I know you should use escrow but just hear me out). He gets tagged by DTs. Two years have passed and he can now delete his feedback and can freely scam again. Would you be willing to make another deal with the same user again or recommend that someone else does it? After all, his negative trust has now been removed.
The only way in which this could work is by taking a look at each case individually. Like you suggested, someone who gets tagged for Bounty abuse or wearing a signature of a scam ICO (without being aware it is a scam for whatever reason) should get a chance to redeem himself after some time has passed if he has shows signs of improvement. But giving everyone the freedom to delete the trust ratings they don t agree with is a dangerous thing.
It depends what angle you look from. I look from the angle where 1000 accounts that don't scam get useless. But I have seen the fight is lost