Please keep this as a public comment instead of making this into a Report a Problem ticket accompanied with deletion of this topic.
You say that the missions are going away because they only exist for Math. There is a simple solution to that one:
Make missions for the other subjects!
You could do World of Science, World of Art History, etc., for all of the other courses.
And as for the archaic code, just write it in a more modern framework!
We would't have the issues that caused people to request the ability to restart/reset unit tests if we were doing missions.
With missions, we got 6 questions at a time, and when one was missed, we had a chance to level up when it showed up in another set of 6, without necessarily running the risk of leveling down on an already mastered skill when we had to do a whole unit test over again and get questions about the mastered skills, too. Yes, questions on already-mastered skills did show up again, even in other courses, and we could level down on them. But the exposure wasn't so great.
Also, when a new skill was added to a course, we didn't have to take the whole unit test over again just to achieve mastery in that one skill.
The worst thing that could happen with the Missions was having to wait 2-16 hours to try it again if someone missed too many questions.