You are slightly wrong. Gold, Silver and Crystal did get an eshop release on the 3DS a few years after Red, Blue and Yellow. Also, whilst they aren’t that common, I occasionally come across copies of the 5th Gen in my local second hand retailer which are all under £30. I don’t think those are old enough to have seen a major price rise.
But on your main point, Nintendo putting more older Pokemon games on the eshop won’t be a bad idea.