It's interesting how each market platform is trying to tweak the equilibrium of honest/dishonest buyers vs honest/dishonest sellers.
Beyond Amazon specifically, the big picture trend is that any marketplace of 3rd-party sellers which includes eBay, PayPal, Uber, Airbnb, etc will always tilt the advantage towards the buyers.
E.g. Buyer scams sellers by claiming he got a bad product and Paypal will instantly issue a chargeback without any investigation.
If sellers try to change the balance of power by shifting to a platform that's more favorable to sellers (e.g. p2p OpenBazaar, craigslist, etc), they'd have to calculate the revenue loss from a smaller marketplace (less buyers' eyeballs). It's going to depend on the type of items if the loss is worthwhile.
E.g. If you're a seller that happens to have the unreleased iPhone 8 prototype to sell, you can use OpenBazaar to sell it because there would be plenty of buyers that don't need refund assurances from Amazon. If the item gives the seller leverage, the seller could also demand more buyer-unfriendly terms such as mandatory payment in Bitcoin instead of Paypal.
When it's not a multi-sided platform, that's when rules can tighten up to favor the seller instead of the buyer. E.g. Costco stops generous 90-day returns on big-ticket electronics and computers because buyers were "renting" video cameras for free.
(Cynical conspiracy hat on: it's possible Amazon expected the sellers' backlash and they don't care because this new policy was their way to cull the "weak" sellers that couldn't take the financial hit from returnless refunds. E.g. The "returnless refunds" policy would drive away shady Amazon Marketplace sellers that scam buyers with faulty USB-C cables: https://news.ycombinator.com/item?id=10508494)