Oh my god so I is closer to ы then e? If it is so, it makes it even more difficult to distinguish when they're speaking quickly. In spelling, there's of course no problem to distinguish between I and ы. But in normal speech yes.
perhaps it could be goodbye after a response to something? For example someone says "Have you got your lunch for school?" and you'd reply with "Yes, good bye" or "Yes, see you later". i'm not too sure but i think that's a pretty plausible reason.