Losing the ability to redeem miles on Delta flights is a small hit. Delta’s transatlantic award availability can be pretty decent especially during low season and on really lightly booked flights.
The real hit is no longer being able to credit Delta flights — especially cheap Delta domestic first class flights — to a Mileage Plan account. It was possible to fly a decent reliable domestic airline in Delta while still earning valuable miles instead of earning SkyMiles.
There’s a big upside to all of this, though.