About the watering out of the Cetra blood - I imagine there were few people enough by the time this process started that, at the game's present, the Cetra blood is not so much faded or disappeared as it is throughly spread out. I'd say everyone descended in one way or the other from one of these Cetra who'd settle down in Gaian groups. This mix would only intensify as better means of transportation were created and cetra-descendants met other descendants and maybe non-descendants and... yeah.
In this way, Aeris' cetra blood wouldn't really fade away if she had children, but as these children mixed with other remains of cetra heritage, they would add extra strength to this common denominator that the human race had been receiving since 2000 years previous. Am I making sense?
At least I felt, from the game, that all humans were potential Cetras, and their current mako-dependant lifestyle was going against not just the planet but their own nature somehow.