Let’s return to the allegory of the river.
Throughout the Land, people discovered babies floating in the river. A few were chosen to save those babies. While many survived, too many babies perished.
Technocrats, Economists, and Statisticians gathered all the Data that they could and discovered that at least 60% of the reason the babies survived or perished in the river was due to babies being tossed in the river; about 10-15% of the reason babies survived or perished was due to the quality of those trying to save babies in the river.
So the Leaders of the Land decided to focus exclusively on increasing the quality of those trying to save the babies floating in the river, saying, “There is nothing we can do about babies being tossed in the river, and there are no excuses for not saving these babies!”
And so it goes…
While this altered tale above reads like a dystopian allegory, it is a fair and accurate portrayal of the current mania to address teacher quality—a mania that simply has the entire reform process backward.
First, the body of research shows a clear statistical pattern about the array of factors influenting measurable student outcomes, as summarized by Di Carlo:
“But in the big picture, roughly 60 percent of achievement outcomes is explained by student and family background characteristics (most are unobserved, but likely pertain to income/poverty). Observable and unobservable schooling factors explain roughly 20 percent, most of this (10-15 percent) being teacher effects. The rest of the variation (about 20 percent) is unexplained (error). In other words, though precise estimates vary, the preponderance of evidence shows that achievement differences between students are overwhelmingly attributable to factors outside of schools and classrooms (see Hanushek et al. 1998; Rockoff 2003; Goldhaber et al. 1999; Rowan et al. 2002; Nye et al. 2004).”
When educators and education researchers note that teacher quality is dwarfed by other factors, primarily out-of-school factors associated with affluence and poverty, Corporate and “No Excuses” Reformers respond with straw man arguments that quoting statistical facts is somehow saying teachers cannot have an impact on students or that quoting those facts is simply an excuse for not trying to educate all students (see Larry Ferlazzo and Anthony Cody for examples of this phenomenon in the debate over teacher quality).
To be clear, however, the problem is not that teacher quality doesn’t matter or that teachers do not want to be evaluated or held accountable. The problem is that addressing in a single-minded way teacher quality is self-defeating since (as the altered allegory above shows) it has the priorities of reform backward.
Teacher quality reform should occur, but it must come after the primary factors impacting learning and teaching conditions are addressed, thus making it possible to make valid and reliable evaluations of teacher quality. That process should be:
(1) Address first and directly the inequity of opportunity in the lives of children to create the conditions within which schools/teachers can succeed and thus school and teacher quality can be better evaluated and supported. As stated in a recent review of misleading “no excuses” and “miracle” school claims: “Addressing out-of-school factors is primary and fundamental to resolving education inequality” (Paige, 2013, January).
(2) Address next equity and opportunity within schools. Teaching conditions must be equitable in all school and for all students. Currently, affluent and successful students have the most experienced certified teachers and also sit in AP and IB classes with low student/teacher ratios while poor and struggling students have new and un-/under-certified teachers, sitting in high student/teacher ratios classes that are primarily test-prep. Inequitable teaching/learning conditions actually mask our ability to identify quality teachers.
(3) And then, once out-of-school equity is addressed and then in-school equity is addressed focusing on teaching and learning conditions, teachers must be afforded autonomy; and finally, we can gather credible evidence to begin identifying valid teacher quality metrics to inform evaluating, supporting, and retaining teachers. 
The first and second priorities can be implemented simultaneously and immediately, with the third priority delayed until conditions are equitable enough to make authentic assessments of teacher impact on student learning. [And regardless, everyone involved in teaching and learning can and must continue to teach as well as possible; that is a given.]
Current arguments that only teacher quality matters are neither statistically accurate nor an effective reform priority.
Current arguments that only teacher quality matters are a frantic effort to save the babies floating in the river while ignoring the real crisis of babies being thrown in the river in the first place.
 See this primer on VAM and VAM-based teacher evaluation reform.