My take: the letter does a good job of stating the grievances and position of the moderators and should be referenced in future articles about the Reddit protests. And, although it's well-written and well-intentioned, it's not going to work because Reddit hasn't been dealing in good faith so why would they start now?
Reddit will mistakenly take this letter as a sign of weakness and not perceive it as what it truly is: a final attempt at communication/reconciliation before walking out. The best likely response would be Reddit making some mealymouthed promises that will never be kept. It's more likely, however, that Reddit will either do nothing at all or do something new that's hostile and foolish. And it's highly likely that the moderators realize all of this, but figure there's nothing to be lost by giving it one last try.