I agree with the comments here about hiring competition - especially in roles that tend to have shorter tenures (call center, gate agents, etc - as opposed to flight attendants who are usually committed a bit longer term due to the competition to get those jobs). I've noticed less of an issue with the call center (perhaps this is the benefit of being a Diamond and getting more tenured agents on average) - where I've noticed a decline (if anywhere) is with gate agents. Poor adherence to policy, surly attitudes, and frequently not knowing how to fix things (e.g., out of order segments after getting protected on a flight).
That said, I don't see a big overall decline in Delta's customer service. Their NPS scores show that they are actually getting better (although of course those can be biased by sampling approaches). The biggest thing I do see is the relative decline. Delta used to be head and shoulders above United a few years ago. United was a mess, so flying Delta was a real relief. The gap - while there is still a gap - has narrowed significantly. United's onboard service and operations have definitely risen steadily after the chaos from the merger.
American is clearly satisfied competing in a different model than United and Delta. Their customer service and focus on customer experience is noticeably less than that of Delta and United.