Have speakers really improved within the last 20 years??

OP,   I will offer this solution for your question. Please forward it to Wendell Diller at Magnepan and specifically reference the 20.1 with the 20.7 Sold today. I am certain he will be able to provide very accurate information regarding the exact improvements and  measurements to satisfy your curiosity.  Unless of course you think it is just some elaborate conspiracy to not still sell the 20.1 since it was not improved?  They simply will not release a new model unless it is a measured improvement above the replacement.  You will find differences in mass, the bass panel itself and magnets. Your ears will confirm with a simple A/B.   I suspect he will view your question and scratch his head wondering if this guy has smoked his breakfast one too many times. 

Can you imagine anything worse than being stuck in a small lifeboat in the middle of the ocean with kenjit? 

By the time anyone found you, you'd be the only person in that lifeboat.

I think I’ve got a line on this conspiracy theory now. We're all blowing it. The cover up isn’t that speakers haven’t been improving and the industry is lying to us. Speakers have definitely been getting better. The conspiracy is that this is being deliberately hidden from just kenjit. Lucky for us his denialism runs deep and he can’t be swayed.

So nothing to see here. Move along.

“It reminds me of that old joke- you know, a guy walks into a psychiatrist's office and says, hey doc, my brother's crazy! He thinks he's a chicken. Then the doc says, why don't you turn him in? Then the guy says, I would but I need the eggs.”

I need new speakers.

Ok I’ll bite.
Your obsession with objective measurement over subjective satisfaction is the problem here. As if you were an oscilloscope rather than a human.
In almost all objective measures, vinyl records can’t compete with high resolution digital files as a storage medium. Yet the emotional and subjective nature of being a human means a lot of people (but not all) prefer to use vinyl. For them vinyl sounds better. There is nothing the oscilloscope can say or do to change that. 
So, are speakers now better at meeting the subjective preferences of a listener? 
Yes. 
Simply because nothing that was available 20 years ago is no longer available. All those designs, materials, and engineering solutions are still around. Nothing has been banned, nothing has been made obsolete by “the man”. All the choices of 20 years ago are still yours to make today.

But new choices have been added in the last 20 years. If you want to listen to a material or a design that is novel, you can.  Just like we now have vinyl and digital storage formats to chose from. 
So, yes, by definition, if nothing has got worse then any change is an improvement. Now, all you need to do is work out if your oscilloscope is up to the job of helping you make those choices.