The opamp back in the days were pretty terrible. NE5532 was the king of audio opamp for decades until the early 2000s.

Modern Class D are built on advanced semiconductor processes (they are considered legacy node in the eye of Hacker News's primary audience. They are at least a lot better than the early days in terms of performance in analog domain.) When an IC company spend a lot of R&D money to develop Class D amp, they for sure exhausted what they can do before they tape out. That results in the superbe performance of modern Class D amplifier.

There is still oppertunities in getting analog Class AB type of amplifier working better, such as adding motional feedback control sensor-less or with sensor. KEF recently released a motional feedback soundbar with back-EMF voltage as sensor. It sure improve the sound quality for a soundbar. Although physics is physics, one cannot make a 1 inch speaker sounds like a subwoofer, but motional feedback sure can make 10 speakder sounds like a 15 inch subwoofer.

Sound reproduction is not just a flat frequency response. Perfect reproduction of phase information generates wider 3D sound stage, without the need of DSP to fake it.