Disney still isn't sure what they want to do with that land 100%. So why would they pay to tear it down, have it sit and vegetation grow in it, only to turn around 10-15-20 years later and have to clear the land again? Now they can just do it all at once.
And the parasite thing wasn't the big driver for closing River Country. crowds had decreased as guests went to TL and BB where there were more rides as well as heated and chlorinated water. There have been several reports that the pump and filtering system of River Country either needed major repairs or did not meet Florida standards any longer. And there were the occasional reports of the parasite. Put all of those things together and it seems like River Country was not bringing in enough revenue to make it worth the costs.