I wouldn’t necessarily say these miners will be useless because if production get suspended due to the network being over saturated (ie no demand) then there will be a window where transaction volume can catch up so if you believe in Dash then things might stabilize and create better returns in a few months and worst case a few years as prices increase. This is why it’s valuable to be diversified and not just mining one coin or at least running some GPU miners where difficulty can’t increase quite like it has for Dash where there’s been a combination of relatively low transaction volume and a massive increase in network rate.
I would say if you can take less than a $500 depreciation on what you paid for the miner then sell it but any more than that and it will depend on your electricity costs. If your electricity costs are low (< $0.07/kwh) then I would just keep running it and hope the network rate stabilizes soon and future DASH price increases retroactively increase your returns. Perhaps a new X11 coin will emerge to give these a boost. With a lot of people taking their miners offline or trying to sell you can expect resale prices to be poor in most places so you might as well hang on to your Dash with some expectations that prices will pop up higher in the next few years. I can see D3s being profitable in places with cheap electricity and I don't anticipate Bitmain, Pinidea, or Inno releasing that much more hardware unless they've overproduced and even then they all likely have mining farms that would be negatively impacted by flooding the network with more miners.
This kind of oversaturation seems to happen often, and Dash does seem to be picking up quite a bit in transaction volume so hopefully that's organic and will continue to increase over time. Profitable mining usually requires a long game that anticipates appreciation of the mined coin over time and an expectation of increases in network rate. If all miners were just dumping when their coins/machines dipped then the market would crash and it hasn't happened yet.
If the D3s represent a significant proportion of your savings then you might be better off writing off the loss and getting into something else because in the near term things aren't likely to be good going into December when Bitmain and a few others are expected to ship several more batches unless Dash user adoption just magically takes off. Now would probably be an ideal time for the Dash "treasury" to start spending big money on advertising and marketing to grow their users in proportion to the hashpower that's coming online.
