The idea of having a number of themed resorts was always part of the plan. You can see this in a lot of the sketches and drawings from the time. Tikiman's website has some excellent materials on the original plans for the area surrounding the lagoon.
It's interesting to look at the building of as many resorts as possible now as a "no-brainer", but at the time, there was certainly considerable risk in doing that. The hotel industry was pretty well established on the coasts, and part of the reason for the selection of Orlando was that it was fairly easily accessible from the tourist destinations already on the map. And as opposed to today, a visit to the Magic Kingdom in the early days could easily be accomplished in a half to three quarters day, what with the ticket system and all.
But as has been correctly pointed out, the more the resort expanded, the more Disney needed to come up with ways to keep people on site; water parks, Pleasure Island, mini-golf, and now Magic Express. On the one hand, Magic Express certainly fills a competitive need, but on the other, if it discourages some from renting a car, it creates even more of a "captive" audience.
Anyway, as someone who has had the opportunity to go numerous times since it first opened, I am always amazed at the level of expansion, and it remains one of my favorite places on the planet - which explains why my family and I are returning in August!