With respect, Course Mastery is not a better experience than Missions, and by a long shot. You should not be removing Missions to make Course Mastery better: you should make Course Mastery function more like Missions to make Course Mastery better.
I have tried to use Course Mastery, and I am sorry to inform you of its fatal flaw: while it is great for teachers who want to easily keep track of student progress and great for students who are trying to cram for their exams, unfortunately it is sorely underwhelming in terms of long-term student retention, as it has no functionality for reminding you to practice material from previous courses again after the system thinks you've "mastered" the course. The World of Math Mission does this, and it is why Missions is an objectively superior tool for learning.
But why listen to me? Because this is coming from someone who made a 5 on both the AP AB and the BC Calculus Tests, received a score of 98th Percentile in Math on the SAT, and is now majoring in Math at a University. I have received an A in Calculus 3, and I am now studying Complex Calculus. All thanks in no small part to the World of Math Mission System which I have been using ever since you guys still had that interactive "web of math" thing. I know how to study math. You guys are wrong here.