If I understand what you're trying to convey, I would posit that it's likely due to the state of the technology. Meaning, speaker/sub design is a known thing nowadays and has come a long way. It's far simpler and cheaper to build a great sounding speaker/sub.
More often than not, speakers/subs sound more similar than different. Sure, there's some subtleties, and maybe what's refereed to as a "signature sound" from a given brand, but they've gotten very close in terms of performance. If you read (or participate in them) the GTG threads, you see this mentioned a lot.
While it can be considered relative, and also dependent on the application/room, the price point for diminishing returns has gotten really low in most cases. One could save up a little more and get something that will last far longer now compared to something from 20, or even 10 years ago.
Like you mentioned about R&D, it takes a lot to make significant advances. look at the JBL M2's (and the company as a whole for that matter) and how long they took to design it and what it costs. Their models aren't updated but roughly every decade.
So the gap in low-high cost models is expanding, almost exponentially. There's almost no more mid-range, or at least you could say it's become the budget area. While the previous occupants have gotten so cheap they are where the HTiB used to reign.
The upper level stuff will go for a lot more but be accompanied with features and support to justify that pricing. Finer finishes, in-home demos, better return policies, better QC on products, more well trained dealers, stronger customer support system, etc.
I'm sure you know all this, so maybe it's for posterity or the uninformed reading this. Or maybe you were saying something completely different..