As an alternative, could you just have it so if I mark an email as spam, any calendar events from it are deleted from my calendar?
We think the default is the right balance for most people, and provide easy knobs to adjust the settings if you want.
Right now marking the email spam doesn't find the related calendar events. There's some design work to be done there around user interface when marking emails as spam in bulk (particularly with support for undo and rolling back the calendar changes which isn't as simple as just applying the attached event again, because you may have updated the calendar event since)
I don't even ask for an auto-add. Just a button. But apparently this is one of the most forgotten feature ever.
Either devs don't think about it, or they tested it and it confused the non tech-saavy users so much they decided against it. I can imagine the later very well, though. My mother would click and the setup for a calendar she doesn't have would start, loosing her the process.