The ambiguous policy upshot of this is precisely what makes intra-progressive fights over education policy so fraught. The exact same evidence which suggests that we should offer higher salaries to teachers also suggests that many of our current teachers are sub-par. It’s easy to assemble a “let’s spend less money on teachers” coalition, which is just conventional anti-tax politics. And it’s easy to assemble a “let’s give more money to the teachers we have” coalition, which is conventional service provider politics. What’s tricky is a “let’s spend more money precisely in order to get different people in this field” coalition.
I think that there is another angle to all of this discussion that is often forgotten. Current teachers include people who sacrificed earning potential for long-term job security (and did so in an environment where this was a part of their explicit employment contracts). The modern vogue for reneging on promises that are not inconvenient is not helpful to the debate. There may be cases where this is necessary, but it should be a painful last resort and not a routine talking point (see state pensions and the rhetoric about them).
EDIT: Also worth reading is Dana Goldstein's column