Firstly, I referred to GaN FETs as a flash-in-the-pan technology, not class D as a whole.
Secondly, I didn’t claim all GaN amps will necessarily perform poorly, merely that I have considerable doubt that any of them are really noteworthy.
Lastly, the primary reason class D has been gaining market share is that more manufacturers are adopting the technology as a way to save on both manufacturing and shipping costs. Unfortunately, many of them choose to not pass the savings onto the customers. That and class D on the whole has improved in sound quality over the past couple decades, and some do indeed approach the performance of decent AB amps. However, the majority are still rather sterile and lacking in dynamics. Thing is, many “hi-fi” manufacturers are merely in the game to maximize profits. If sacrificing 10–20% of performance vs an AB counterpart allows them to do that, you can be dam sure that’s what they’re going to do. They also know that the majority of people purchasing these amps don’t have a decades-long history of obsessive gear flipping. These are often people looking for a simple, lightweight and unobtrusive solution with impressive specs on paper, and have little reference by which to judge performance. That and auditory memory is generally unreliable. So unless the listener A/Bs their latest/greatest class D amp against their past class A-AB amps, it’s unlikely they’re going to notice the shortcomings, and just as likely their expectation bias will lead them to a skewed conclusion.