I think this is very community specific, by your own admission unintended collisions would occur. What value is there in having a smaller number of much longer topics?
For communities such as meta this would mean that any user who emails in with ‘problems configuring mailgun’ will continue to be notified of updates months or years after solving their issue. That doesn’t sound practical.
The goal is to avoid duplicate titles, which the current setting enforces for topics created on-site as the user can be given feedback at the time of topic creation. The setting isn’t respected for users emailing in because there’s no such opportunity.