There is a small but measurable anticipated demand for these old shares from people who owe loans payable in shares (people who sold DL stock short at higher prices). Hence, they have a reason to trade, and to trade above zero. Once these folks have repaid all of their loans, the value should head straight towards zero.

Once the reorganization (or liquidation, if that's the outcome) plan is approved by the court, the "old" shares are canceled and worthless.

In very rare cases, there are enough assets relative to debt that a tiny fraction of the "new" shares (perhaps 1%) gets allocated to "old" shareholders. There is zero chance DL's case is one where this will be the outcome, as the unsecured creditors stand to receive only 20 cents or less on the dollar for what they're owed.