Many incorrect assumptions here.
I don't ever want email from moodle.org. I will come here when I want to read messages.
Therefore, i have turned off auto-subscribe in my profile, not because of some missing feature, but because I don't want email.
I suggest making the user profile option have three states. When I post to a forum:
- Don't subscribe me
- Subscribe me to that discussion
- Subscribe me to the whole forum.
There is then the question of how you make that discoverable.
Ideally, it would be like new admin configuration settings. When an admin comes to the site, they are prompted to set any new settings.
Similarly, we could give a pop-up for users, asking them to chose the default. If we were really clever, we would to that contextually. E.g. the setting above related to posting in a forum, so when you go to post in a forum, if you don't have that setting set, then the forum subscribe option has no default (Choose ...) and becomes a required field. That sets the default.