You only give someone the ability to override roles when they understand how roles work. 99% of all teachers do not understand role internals, and will never do so.
Making the default Allow sends the wrong message. It says "overriding is easy – anybody can do it, even teachers!" But as I have learned in many months of helping administrators on moodle.org Roles and Capabilities forum, not even administrators understand overriding.
By making the default 'Allow', teachers will be able to override roles starting on Day 1. We can expect double the number of posts from users who have gotten themselves into trouble, not just administrators, but now also teachers.
Until teachers have gotten roles training, they should be restricted to USING roles and should not be allowed to fiddle with role internals. Administrators, if they are foolish enough to do so, can edit the default permission. Smart administrators allow "safe" override selectively for teachers who have received roles training.
By allowing teachers to override and by adding roles to the (formerly empty) list of roles they can override, Moodle has swung from one extreme to another. The so-called Risks which you considered do not take into account "training risk" which is significant.
This is the worst decision I have witnessed in two years of moodle.org. And it was done without any discussion. For shame!