Well, those inefficient speakers will flourish with more powerful and higher quality and affordable class D amps.
Maybe. But its a simple fact that with less efficiency comes more thermal compression. Its also a simple fact that if you make a power amp (any power amp) work hard for a living, its going to make more distortion.
That additional distortion is often audible as harshness and brightness. We've been hearing that for 60 years... IMO/IME if you want the best performance out of your amplifier dollar investment, its best served by speakers that are of higher impedance (8 Ohms being better than 4) and also easier to drive.