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I'm 99% sure it won't have much of a positive impact in sales simply because the majority of ebook buyers don't care about DRM anyway, only the minority of us bother to strip DRM. So while it wouldn't be a large drop in sales I do think it would still be a drop. It might not be enough of a loss for them to care, and tbh as you said it would probably only result in a mild increase in piracy while the majority either do library loans or switch to paper.
I don't think that those of us who care enough to jump through the hoops to strip DRM are just gonna roll-over and accept that the publishers can yoink our entire libraries whenever they see fit, but I do admit most don't care. However those that don't care aren't stripping DRM anyway, they are just relying on the ability to redownload their books whenever they wish from Amazon/Kobo/Nook.