Delta has, without question, taken their eye off the ball when it comes to actually running a top-of-class operation. I've seen a steady decline over the past ~4 months as traffic has come back. More delays, more miscommunications, more poorly handled IRROPS, staffing issues throughout the company.
I wouldn't say they are terrible (with the very notable exception of call center response times, which continue to be abysmal) ... but they are right in the middle of the pack now. In other words, no reason to pay the premium anymore.
If it wasn't for Singapore (which I now have on speed-dial) I would likely not be flying Delta at this time. So Singapore saves the day for this flyer...