I think you're overstating things. On the one hand, a lot of applications where quality wasn't that critical switched over ages ago. And, on the other hand, any application that would have spent the money on voice acting is still going to pay for both the higher quality and for a sound that isn't the same as everyone else is using. (Note that Siri's new iOS voice is based on a new training set from a new person.)

I do think there are applications that we don't just have today because TTS just isn't good enough. I've had some ideas around Alexa apps related to content that would be TTSd. But the current Polly just isn't human enough. I don't think this is there yet either but it's getting close.