If Susan can make the edit from memory, we're good.
No, we're not good. That's the whole point. People's memories were reliable enough when Wikipedia first started and nobody actually used it for anything. That's no longer the case. If we want to be a credible encyclopedia, we need our facts to come from reliable sources (citing them isn't the important part, that's just a way to prove the important bit - that the fact came from a reliable source).
There seems to be a serious misunderstanding of what "source" means. A source isn't somewhere people can go to verify the fact, it is where the fact came from. Citing sources is easy, because you will always have the source with you when you write the article (if you think you don't, then it means the source is your memory, in which case you aren't using a reliable source and shouldn't add the fact). The problem isn't that people aren't citing reliable sources, the problem is that they aren't *using* reliable sources.