One of the main motivators that keeps me coming back to this website is this mission specially. It is extremely satisfying to see how many blocks you have filled versus how many you have left. For example, this is what mine looks like:
Seeing that everyday legitimately makes me proud.
With the new system, you lose the ability to really see just how far you've come. Sure, you get percentages based on each specific subject, but you have no sort of visual that shows every individual skill compared to one another in one central place.
This new system is not enough by itself. For example, I usually aim to get 100% on every unit test so I can have the entire unit at level 6 with all skills mastered. However, considering there is still no button to RESET the tests, I won't even bother to continue and restart the test if I make a small mistake early on. This usually occurs when the test is 10+ questions long. With the mission system, I'm able to target the skill that was with the question I messed up on and master it immediately. This gives me the satisfaction of having another block filled, level 6 on the unit, and I'm no longer stressing about having less than 100% on that unit test.
The missions and mastery system work well for the reason above. Since if you master a skill in a mission, you no longer need the unit test to master that specific skill. All the new system is doing is putting far, FAR more importance on the unit tests which is the most stressful part of learning things if you are OCD about having 100% on everything.
If you won't keep The World Of Math or offer a replacement, then I strongly urge you to atleast add a reset button to unit tests instead.