A number of factors are conspiring to make this hobby less interesting and more difficult to sustain. The main lines from Mattel for Hot Wheels and Matchboxes are getting less collectible.
The base range is still less than $1 retail in the US. In order to keep this price point, Mattel has resorted to moving assembly to the cheapest locations possible, and also attempting to reduce the cost. To that end the zinc alloy is being used less and less, and plastic more and more. Most cars have a plastic base and a zinc shell. Some have a zinc base and plastic body. Adjusted for inflation, the 55 cents in 1969 should be almost $4 retail today.
There seems to be fewer recognizable models in the lineups. Models that resemble real cars and trucks have to be licensed with the manufacturer. This adds to the cost. These are the most interesting models. The Generic models are often out of scale and impractical looking. Matchbox has been following the Hot Wheels pattern since the 1970s in this regard.
The retailers are reserving less shelf space for these cheap die cast metal cars. I would assume they need to produce a certain amount of profit per square foot, and at $1 a car it does not work for Target, WalMart, KMart, and Toys R Us to devote retail space for low margin items like this. At $4 a car, there could be a higher margin per item. Perhaps the next step will be to reduce the packaging card size to allow more pegs on the display board.
The interesting models are being snapped up by the speculator or reseller crowd. The only place to find a classic Seagrave pumper, for example, is on eBay.