A problem with this is that post attribution to unique users will be lost, so it will be hard to follow a conversation where all posts seem to come from a single system user - you won’t be able to tell distinct participants apart from each other.
Exactly; conceptually, you could either leave things as is and manually use the admin user profile UI to merge users that should be the same ones, OR you could simply send email as external ID instead as you suggest, which will new logins to be connected to an existing account with a matching email address.
The latter is clearly the least resistance path 