It's a fair point, but by the same token - are the 1-star ratings harming sales? Outwardly, the obvious answer to that might be 'yes', but we have to judge readers based not on what they say, but on what they do. If you're not selling and you have an abundance of reviewers howling about the apparent lack of editing, then you might have a case - but even then, it's not a sure bet. And, if we're telling would-be indie publishers they just "have to" pay for editing then in essence we're saying it is part of the "sure bet" one has to make to see sales success, and I think that's misleading.
The problem is that we don't really know what readers do. There's no way for us to measure how many people glance at bad reviews and then move on. Even a book that is selling fairly well might have been selling twice as many copies without the one-star reviews bemoaning editing. I take your point that a book that's not selling might not be in trouble because of the one-star reviews, but your logic can just as easily be applied to your side of the argument as well. Just because someone self-edited, and the book is selling well doesn't mean that the person couldn't have benefited from a professional edit.
How many books have been subject to a professional edit that also have been 1-starred because 'x' reader perceived the writing to be of low quality or to have lacked editing? How many times has an author themselves edited their own work to the nth degree only to be met with scorn from certain readers pertaining to the editing or perceived lack thereof? How many times has an author launched with just their first draft and gone on to make a killing?
There's a myriad of answers to all of those questions.
I'm sure it happens, but in order for that argument to really carry weight, you'd have to be able to show that it happens a lot. I'm not persuaded that a freak accident kind of scenario is a reason not to get a book professionally edited. Sure, some readers' perceptions of whether a book has been edited well or not can be flawed--but that's true of virtually every feature of a book. We could just as easily say, "There's no point in spending time on character development, because I have, but reviewers still criticize my characters." Anyway, if a book really is poorly edited, it's probably going to attract a lot of one-star reviews, not just the occasional idiosyncratic one. Is such a pattern likely to affect sales? We know there are writers on this board that would take a pass immediately. I know a lot of readers who get bent out of shape if a book has more than a certain number (usually small) of editorial problems. They may not return that book, but they won't be buying more by the same author any time soon. I know zero readers who don't care about editing at all. Maybe the people I know are all idiosyncratic, but I doubt it.
Part of the problem I have with paying healthy sums for editing is, there's no way to tell from a buying customer's perspective whether the book's been edited or not. I can't pick up a copy of you-name-the-famous-novel from my shelf and tell you whether that's the author's 6th draft or her 27th - whether her work was gone over by just one trad house editor or several. Just as there's no tangible way for an author/businessperson to tell whether their sales success has had anything to do with the money they shelled out for an editor versus had they not spent that money and made more profit on their book(s) because whether they knew it or not, as far as the market was concerned, their writing was 'good enough'.
A buying customer who has good language skills can tell from a quick glance at the Look Inside whether a book has been adequately edited or could have used another pass (or perhaps several). And that's all a customer needs to know. How many drafts or how many rounds of editing a book has had is irrelevant--but the finished product isn't irrelevant.
It's true some people will complain about work they perceive lacks editing, but it's also true there are books all over the bestseller lists that could easily appear to you and me as being in need of a professional edit and yet the market is telling them, and us, something different.
No book is perfect, but I've yet to pick up an NYT or other bestseller that looks as if it needs a professional edit. If you have any actual examples to cite, please do so. As far as Amazon lists are concerned, we all know those can be--and are--often manipulated. A book could be ranking very well on Amazon as a result of ghost borrows or botting and not really be doing that well in terms of real sales.
I would say, if you've spent lots on a great, commercial cover, and a lot on ads, and you're in a popular genre with a kickass super-commercial premise/idea, and your blurb's on-point AND you're not selling...then maybe the writing's the issue. But, would a professional editor help you there? Which one? In what capacity? Was the issue your syntax, your grammar, or does your voice just suck? How many copy editors and how many hundreds of dollars later would it take for you to find the answer? How many developmental editors will agree on what your problem is? How many of them would any given author have to pay to iron out their craft issues that may or may not result in weak sales once a product is put to market?
Yes, editors do vary in their responses, so one needs to use their advice selectively. I thought we were talking at least partly about grammatical issues, and there's less room for argument there. Anyway, your reasoning, if generalized, is a recipe for inaction. One could easily ask the same questions about cover design, advertising, or any other element. For instance, no two cover designers are going to create the same cover for your book, just as no two editors are going to have exactly the take on your manuscript. Is that an automatic reason to do your own cover, even if you have no artistic or graphic design experience? Hardly. As far as the rest of that paragraph is concerned, yes, there are no certainties, and even some great books doubtless don't become great sellers. That said, doesn't it make sense to make the book the best you can reasonably create with whatever resources and talent you have? Whatever else someone might claim, I doubt anyone can claim that producing a better product is going to hurt sales.
There's so much amorphous intangibility surrounding the idea of editing that it makes me reticent to recommend to anyone in this business to commit dollars to it, let alone hundreds of them. I just don't think there's enough proof-of-concept in terms of directly linking editing to healthy sales numbers that would lead me to support it...that is, outside of anecdotal evidence or the promotion of the idea of "well, the traditional publishers have been using professional editors for 'x' number of decades and look how many billions they've raked in therefore..." arguments which aren't really rational arguments in my view at all.
And again we're back to questions we can't know the answer to. If you want more than anecdotal evidence (which is at least better than no evidence) for anything in self-publishing, you may be waiting for that forever. If we had a comprehensive study that included both editing processes and sales figures for a cross-section of authors, then we might be able to draw conclusions. Until then, anecdotal evidence is all we have--and it points to the idea that a reasonable amount of editing is usually necessary. That doesn't mean spending big bucks to have it done professionally. Some people really can self-edit. Others can find low-cost ways to do it. I don't think there's any one formula that is right for everyone. However, I think newbies need to develop the self-awareness to know whether or not they need editing help--and how much. From my (admittedly limited) experience, "the best one can do under the circumstances" is a better bet than "good enough."