I was just complaining about this problem today. If I'm CCed on an email exchange and 4 or 5 messages go back and forth then the problem is solved, i still have 5 unread measages in my inbox. In reality, I should have zero messages because the problem has been resolved but SMTP doesn't allow for that.
The problem with this approach is that it leads to everyone reading every email just in case they might have some relevant insight. I see this at my current job - there's a strong "just cc the whole team" attitude which leads to a frustrating quantity of irrelevant email.
Related to this is a common corporate problem where someone has a problem they perceive as somewhat urgent. They often send a message to several people, hoping to get the first possible answer. Sometimes the recipients experience diffusion of responsibility and no one answers, because each believes someone else is better able (or less busy).
There are systems with retraction capabilities for senders (e.g. Bloomberg MSG, though it is no competitor on price). But I haven't seen anything like a system which would downgrade messages already seen by others, etc.
