For what it’s worth, I recently created a plugin that accomplishes it. Maybe eventually it can be pulled into the main app rather than being a plugin. Unfortunately, the plugin doesn’t allow for hiding of the original ‘inbox’, ‘sent’, etc, so these folders are going to be duplicated in your view; AND I wasn’t able to get them to appear first in the list, but I added a preference to the plugin that allows you to auto-select the ‘filtered’ folder so it ends up being the default one to open.
I wrote the plugin in an afternoon, and have only been using Mailspring for a week, so there’s no guarantee it’s bug-free, but at least you can go back to the normal inbox as a fallback.