Revise naming to provide a more user friendly naming structure (Newsletters => Mailing Lists; Newsletter issues => Announcements)

What you say is somewhat true, although everyone will have different ideas on the best name. We could check what some of the mainstream mailing list providers do.

• Rename "Newsletters" to "Mailing Lists": I would tend to agree
• Rename "Newsletter issues" to "Announcements": I'm not sure, as it won't always be an announcement, it could be something quite different.

However we have some problems to change things (and I expect the failing tests relate to these):

1. If we change route names, config parameters etc it will break existing sites. At most what we can do is change the human readable UI text.
2. If the external names don't match the internal ones (e.g. Newsletter.php defines an entity of type simplenews_newsletter), then that would also be confusing.
3. People have got used to the current names, and some of them will be confused if things change.

I'm not sure how much we can do here.

OK with the new 4.x branch we have a chance to make some non-BC changes.

URLs should use the module name to avoid clashes, and in 5 out of 6 cases from the issue summary we do that already. I would support this change:
"/newsletter{/...}" to "/simplenews{/...}"