There was a very strong following, especially in the educational market. I remember seeing schools purchasing labs of IIGS's as late as the early 1990's.
Basically, the apple ][ was the cash cow that kept Apple afloat for years while they tried to sell 68k macs. Apple basically tried to kill the II for a decade but wasn't successful enough to just cut off the customer base that was crying for new models.
Epic 1: The Apple II sold with no expansion cards, but many expansion slots. Hackers and business designed addons for years.
Epic 2: the Apple IIe (and later IIc) were sold with an optimal set of expansion cards.
So you had one generation of experimentation and a second generation that leveraged all the hard work!
Hackers and "business" continue to design and sell cards for them!
(CompactFlash & USB-storage interface card) http://dreher.net/?s=projects/CFforAppleII&c=projects/CFforA...
(ethernet boards) http://a2retrosystems.com/
(RAM boards) http://www.brielcomputers.com/wordpress/?p=321