An alternative would be just to be able to select your "default" workspace. A simple checkbox at the top of each workspace could be "Make this my default workspace". That would meet my need just as well as "persistent" workspaces.

Closing out officially with a "won't fix" status. I believe this was dropped as it's no longer relevant once workspaces changed locations. Starting with 4.1, workspaces were moved into the 'home' menu and are now divided into individual tabs, next to the 'Notifications' tab. Because the 'Notifications' tab is always the default, it's impossible to have a second default workspace tab selected.

Note that when this request was written it would have been possible and is why it was endorsed by the developers. Because at that time workspaces were in their own self-contained section of 'mail' and the current workspace was selected from a single dropdown, making that selection persistent (or the default) was a real possibility.

note: I will update the linked threads with the news.