With DL keeping their middle seats open until at least the first week of 2021, they are selling about 30% fewer seats than carriers like American and United, which are booking all seats to capacity. AA and UA can sell more seats at a lower fare, while DL may need to charge more, per seat, to generate the same revenue on the same route, same size plane. This also means that DL's lower fare buckets are selling out more quickly, than other airlines which are not blocking middle seats, as DL has fewer seats to sell, right now.
It's somewhat reassuring to block middle seats on flights, but someone is paying for that option.