Solution has to be multi-factorial since so is the problem, much as I love Val's observation and think it is spot-on:
All of the flawed thinking leading to these approaches comes from the same well of ignorance.
(FRAME-WORTHY.)
Anyway. The solution is to teach math BETTER to begin with-- and the only way to do that is for K-8 teachers to
understand the
numeracy skills that they are trying to teach. Not "know" (as in "memorize") but UNDERSTAND the underlying principles. (Which, to be blunt, many of them currently DO NOT.)
So that is the first thing.
The second aspect of that is to junk curricula that are pure garbage-- like EDM and its ilk. Curricula that are mathematically SOLID probably ought not be written by
educators, but by people who are actually mathematicians themselves...
teaming with educators that understand child development well.
Okay, so that is how to improve teaching-- unfortunately, CCSS did the former, but then turned over the latter without actually making sure that the people writing the daily curriculum offerings understood what those things actually
meant. {tears at hair} It's not that we don't know what this kind of "what kids need to understand in order to gain fluent numeracy" looks like in reality-- subject expert mathematicians HAVE told us. It's just that the educators and particularly the publishing industry... isn't LISTENING. Or if they are, they are listening the way that my dog might if I tried to explain, say... one to one correspondence to him, or the significance of negative values. He can hear the words as sounds, but not meaning. While I realize this sounds mean-spirited, it's about the only explanation I have, short of rabid conspiracy theory, for what has come of the very reasonable and rational standards in CCSS, as opposed to what I'm seeing in classroom materials and pedagogy.
All right. So without fixing K-8
teaching and curriculum, I think that there IS no fixing this problem.
Currently, students on one end of the ability curve are shortchanged because things are far too over-simplified, watered down, sprinkled thinly, and not rigorous enough... and on the other end, we've mashed concepts into a puree that we can spoon feed everyone... only, because it's been pureed without any intentional plan (well, seemingly so) it leaves them so malnourished cognitively that they are incapable of being functionally numerate when they reach secondary math topics. If I had a nickel every time my DD saw a student in HIGH SCHOOL GEOMETRY OR ALGEBRA who did not actually understand
fractions, I think she could have a tidy sum-- probably enough to purchase textbooks for college this next term. Those are kids who have been deemed "adequate" in their understanding of K through 8 mathematics. Holy mother of turtles, but there is something VERY wrong when not knowing how to write an equivalent fraction is still "okay" at an 8th grade level. No wonder they can't understand geometry. They don't have any CHANCE of understanding it-- their foundation is like wet cardboard.
So I vote for mastery-based learning, which in practice probably most resembles tracking with in-class differentiation-- but all of that only AFTER a complete curriculum and pedagogy overhaul in K through 8-- and that must be directed by actual mathematicians, not by those who are edumacators playing at math teaching.
Rather than standards that get twisted and warped by people who are ignorant of the larger math concepts that they are distorting out of recognition, there needs to be a more iterative process in developing math teaching at those lower grades-- that is:
1. Standards come from math experts
2. Teaching/child development experts assign what they feel is appropriate timing and methodology for teaching those standards,
3. classroom teachers provide input into what they know works in classrooms with live students,
4. math experts tell them what they have gotten wrong (which will ultimately come around to bite students later), and
5. repeat points 2, 3, and 4 until the math experts give it a passing grade.
Clearly, elementary educators, administrators, and elementary teaching colleges are NOT going to be pleased by my plan, since it kind of points out that they actually need both hands and someone else to hold the flashlight, but anyway, it's what I think.
I also think that this problem exists to a lesser extent in all STEM subjects-- the number of children who graduate from high school without being able to actually explain what a hypothesis is in their own words, or to simply state purpose is served by a "negative control" in an experiment? It is astonishing. These are kids who
have taken AP coursework. 
It's the teaching-- the kids aren't STUPID, they just have been taught stuff that is profoundly ignorant, and they've been taught from day one that STEM is about
memorizing, not understanding.