In my opinion, back in the 1980's and 1990's, the technology was still evolving, so that each new model did bring a significant increase in sound quality and features. But now, we're at a point where CD-quality (and beyond) sampling capabilities have become dirt cheap, so that any improvement in the sampling rate or number of bits sampled would be essentially inaudible to the human ear. It's kind of like adding more colors to a graphics card in a computer. I once read that the human eye can distinguish between 8 and 10 million different colors, so if we already have graphics cards with 24-bit resolution (equating to over 16.7 million colors), how much can we gain by increasing the number of colors, even if it is technologically possible?
When you're talking about $5000 keyboards that are already loaded with features, it's hard to imagine what more can be done at this point to significantly improve the sound. And new tones and styles can easily be added to existing keyboards. I'm not saying that nothing can be done, but I don't think we'll see the leaps in improvements that we used to see.
With lower cost keyboards, such as the PSR-E473, it's a little bit different, because these keyboards don't have "everything" in them due to their price, so it's always interesting to see what previously out-of-reach features now get incorporated into the new lower-priced models. Back in 1990, it would've been unheard of for a $370 keyboard to have over 600 tones, synth features like filter and envelope generator, dozens of digital effects, and sampling -- all at or better than CD quality.