Why should someone who has doxed someone get away with it by deleting their account?

Doxxing is not illegal in many places - the US included. Cyberstalking and harassment may be illegal, depending on location. That's beside the point, but this is an extremely specific example.

Ultimately users should, in my opinion, be in control of their data. Tildes, for example, preserves deleted comments for (I think) 30 days and then permanently removes them. It seems like that approach is a compromise that would work for your situation while still respecting privacy long term.

So the key thing here is, "are you aware that the data is part of a legal proceeding or crime?"

If no, deleting it as part of normal operations is perfectly legal. There are plenty of VPNs which do not log user information, and will produce for the authorities all of the logs they retain (i.e. an empty log file).

From an ethical standpoint, keeping peoples' data which they want removed, against their wishes, based on the hypothetical that at some point someone might do something wrong, is by far the less ethical route.