Seeing eight open D1 seats in the seat map doesn't mean that there are really eight D1 seats available. Some passengers might not have selected seats, etc. Also, schedule/equipment changes often result in passengers without seat assignments until they're aware that they must re-select a seat on the map. However, if you try a dummy booking for D1, what fare class appears? How many seats can you (dummy) book in various fare classes?

It's possible that there could be some GUC upgrades to D1 on the day of departure, especially for people who bought coach and were already upgraded to PS. That might be why the agent is telling you that you'll get a PS seat at the airport, although I'm suspicious that you see the downgrade and coach seat assignment rather than some version of confirmed and seat to be assigned at gate.

Are there later flights on the same day with PS/PE availability to which you could be moved?

Can we assume that you have no DL/SkyTeam status? Did you pay for the entire ticket (except taxes and fees) with miles or did you use miles to upgrade a coach ("Main Cabin" in Delta-speak) ticket purchased with money?