Are future improvements in Amp/PreAmps slowing to a crawl?

Yeah, but class D amps are just attempting to solve a problem unrelated to sound, namely, power consumption. Not everyone wants a big class A amp that runs as hot as a toaster. The problem of flat, low distortion amplification has pretty much been solved in a variety of ways which wasn't so much the case 40 years ago. Back then even solid state watts were pretty expensive. 
I never expect to own a class D amp. I've got a healthy Pass F5 clone belting out about 37 watts RMS and it's probably the best sounding amp I've ever heard. I'm not at all bothered by the fact it runs as hot as a pot of coffee. 

Call Underwood hi fi..big stuff coming out early next year. 

Class D is second rate sound quality, not in SOTA league at all!

Class D is on the cusp of breaking out into SOTA performance.  There are a very few very expensive Class D's that can really strut...most are very far away.  I remember when CD players were awful....now, there are some excellent ones. 

I liked the closing of the article. He mentions without saying bluntly, that most of the designs out there today are spinoff’s. Dashes of this or that thrown in, and a new something is born. This is what I took from it anyways, right or wrong. Yes, this happens all the time. Most of the topologies out there now are repeats. I mean, how many ways do you think you can run a tube? Transistor? Wouldn’t one think those kinks were ironed out years ago? I sure do....

I knock people’s heads off quite regularly with a few of my restored 30+ year old amplifiers. Yes, they are solid state, and they sound nothing like the type would suggest, nor do they sound like tubes. They sound like music. If something sounds solid state or like tubes, it is not right IMO. I have not heard any revolutions in amplification/preamplification with anything new, to be honest. "Who is this clown?" you say? Who cares. Let’s just say I have been in this hobby all my life, and I have had a ton of gear, with a good set of musically trained ears.

Now, DAC’s are another story. Cables are another story. Electrolytic capacitors are another story. Everything else however, not so much..... All of course, JMO.