The OP is correct in that the Saga (so far) showed us those things, and that Lucas himself said those are the reasons why no more movies would be made. But as others have said, that is what he said at one stage of his life. At other earlier stages he said he wanted to make this a 9 or even 12 part series of movies. Granted, any story treatments he had for the third trilogy were reportedly squeezed into ROTJ, but I think there must have been more to it than what actually fit there. At one point, many people were happy to have Star Wars (ANH) as a standalone movie and thought the story would (or even should) end there. Most of us would agree that it only got better when it became a trilogy, and many of us also think it got better again when it became a six-part series. So as long as they do this right, there is no need to think anything will be ruined. The Saga can grow again and get even better.