I think the "market size" is most likely increasing in the R/C plane hobby. This is due to the fact that a lot of people are not builders, so the relative ease of entry into the hobby today via easy-to-assemble ARFs is a great incentive for those that would not otherwise enter it, to get involved.
Now, I have not been involved for 12 years, (up until 6 months ago) so I don't know how long the trend of well built ARFs has been around, but I do know that it takes time to build the base necessary to bring prices down. This is exactly what happened to PCs. I remember in the early 70s the Commodore VIC 20 computer... It had 20k of memory, and sold for $800! During the 70s, 80s and 90s prices dropped as the market for these "toys" increased. This is 3 decades of demand increase.
Can we expect the same of R/C "toys"? It's obvious, and it is realized with the example of R/C radios listed above. I remember the Kraft radios, the OS/World Engines radios, etc. etc. They were EXPENSIVE considering the relative income of yesterday.
Perhaps the general complaint is ARFs. How long have these quality "new" planes been around? 5 years? If I'm not mistaken, those first ARFs from Hobby Shack in the late 80s and early 90s were crap, and they were $100 plus! Today the $100 plus can get you a much nicer plane, and the $100 is actually worth less than it was then!
Is it motors? Hmm, I seems to recall my first Saito 120 cost over $300. Maybe it was $350? Four months ago I spent $350 on an improved motor, a Saito 150. What's with that? My dollar is worth less, but I get more! Cool deal.
Is this because the market is getting better? Probably. Is labor cheaper? I doubt it. Supply and demand. Demand increases and prices drop. More demand means more equipment to manufacture goods, which makes the process less expensive for the producer, and to compete the manufacture lowers prices, or more importantly fails to raise prices as the value of the dollar decreases.
It's all good people, and it is these things discussed that will continue to fuel the hobby's growth which keeps prices reasonable and well within the budget of the average working Joe. 30 years ago, the "average Joe" couldn't afford a radio, and a "large" motor, and a plane, so he flew Midwest Super Sniffers with Cox .049 motors free flight. I remember, as my father was the "average Joe" then.
Today's average Joe probably has 2.75 flyable R/C planes. That's pretty good I'd say.