I'd rather have a much smaller set of verified reviews than a bunch of fake ones. Decreasing the quantity in this case often means improving the quality and signal to noise ratio.
There is a difference between reviews on Amazon, where limiting to purchaser makes sense, and Goodreads that is open to everyone.
And yes, agreed that Goodreads needs a different system. I was responding to a post about Amazon, and even there mentioned that goodreads would need something different, maybe reputation based or whatever.
Not sure it would really improve the signal to noise ratio.
It would be thought of as an advertising expense to buy books from the platform in order to qualify to write some of the first reviews.
Even though it's an imperfect barrier to review quality, at least it is a barrier versus none, and time and time again retailers who've experimented with it have found it a useful signal.
Yes, you'll lose out on the borrowers who fell in love with a title, but they can always spend the $20 to buy that book to review it if they really want to (I've done that for things I loved, just to leave a review). But even if they don't, it's still usually a net win: you lose not just the honest reviewers but the 100x spam that outnumber them.
When shopping for low-cost products these days, it's not the individual review that matters anymore, just the aggregate ratings. So it doesn't really matter if a few borrowers don't get to leave their rating, it's still more useful for buyers to cut out the spam.
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Maybe a simple compromise is to just allow filtering (and adjust the rating) by "verified purchasers only", so you can see both the unfiltered and filtered scores. I'd just leave out the unverified ones altogether, myself, but others could see the raw scores and reviews if they wanted to.