Too simple.

From what I have observed of ARF consumers (I'm a builder so have only secondhand knowledge), they "go through" aircraft much faster than builders. I don't know if this is because ARF availability encourages them to buy beyond their skill level or, if the ARFs themself have a built-in expiry as a result of poor fuel-proofing, weak points in design that lead to early failure, or the owners have little sweat equity invested and simply tend to mistreat or not maintain them as a result ... from what I've observed, I suspect a combination of all of the above.

A better metric would be the availability of new kits (not old stock) and building supplies, this has definitely declined.

At the LHS, the racks of materials, tools and kits have been replaced by the larger full colour printed ARF boxes.
Mail order, the new LHS , is not advertising or selling many of these same materials either.