I too am in the camp that think DL silver is worthless. But the OP can do his own math:
Cost: how much for a flight to earn 5600 MQM? That is at least 1 cross-USA trip. I figure at $0.04 per mile (bargin), you will pay $224. This does not include the cost of your time/energy for ~10 hours of flying + logistics. There is also cost of gas/parking/uber to get to the airport. If your mileage run involves overnighting somewhere, then factor in hotel cost.
Benefit: How many flights will you take in 2018? I think you already get free luggage with certain Delta AMEX cards, but assume you have to pay $25 per bag per trip. You would need to be flying on at least 5 return trips to total $250 to offset the cost of your mileage run. Then, there is the fuzzy math of quantifying the worth of the POSSIBLE domestic upgrades. If you tell us where you are based and where you typically fly, we might be able to give you a guess as to your possible upgrade percentage. But I can tell you in advance that if you fly to/from a Delta hub (DTW/ATL/LAX/SLC/MSP/JFK) to another major city, your chance of upgrade is near zero (with the exception of the flight being near empty..i.e. flying on Christmas day) I am based in DTW, and my upgrade percentage over the last decade is near 50%. I've been PLAT or DIAMOND (half and half) during that time. The times I get upgraded are flights to really small airports and. In recent years, at PLAT, I almost never come close to top 5 on the upgrade waitlist when flying to another big city from DTW.
The other fuzzy calculation is what those domestic upgrades are worth... If your flights are mostly short flights in small planes, and you aren't super-tall and need the leg room, then first class is near worthless.
Another weak benefit of Silver is that you get to board a little earlier (Zone 1) than others.