This is not necessarily dumbing down. I am no fan of Common Core. I consider it a wretched piece of crap that is designed to control our children. That said I simply suggest to you that the things ‘removed’ from the K12 curricula are not that big a deal. There is no good reason to teach fractions before 5th grade. There is no good reason to teach probability at all until college. So what if a high schooler cannot take calculus? Most of the calculus taught in high school is worthless when you take college courses that really need it (like physics and engineering).
When I went to school (back in the dark ages) we studied none of this junk until college...and of course it was so detrimental that my generation got to contribute to some of the best growth years this country has seen in modern times. We helped get us to the moon. We improved measurement techniques. We did the engineering and science work that has provided the modern world with many of its conveniences. Not a one of us had probability in elementary school or learned the fundamental theorem of arithmetic. So much of the ‘new’ math is nonsense. It is geared to protecting children from learning their ‘tables’. They use calculates and cannot make change if their life depended on it.
IF the common core would focus more on actual useful skills (can you add and subtract? multiply and divide?) it would be a better thing.