Considering that you are requesting this for web-posters, who typically do not prefer email, this would be all right ONLY if members could opt-out of the extra mail.Perhaps web-posters would have a web copy, or the choice of email and/or web copy.
It could be threaded within the Topic (only that poster could see it).
It should be up to that poster whether or not to ever delete it from their view of the Topic.
Perhaps email-posters would get an email copy, or the choice of web and/or email copy.
Again with the ‘delete from web thread’ options.
Or should web-posts (and probably email-posts) be archived in a separate Post Archive section for each Group of each Subscriber/Member?
Seems like a lot of redundancy to me, but maybe it isn’t a big deal to do…