Maybe the reason Prime economics have become tricky is because Amazon bundled in a video service nobody wants since 2011, leveraging one business' extreme success to juice the numbers of one that's faring poorly against its competitors. Netflix charges $95.88 per year for a similar service. How much of Prime's price hike was really to help pay for the video service that's just a tax on Prime members who have never used it and don't want it?
This isn't just an Amazon problem. In the last few years, Google, Apple, Amazon, Facebook, and Twitter have all made huge attempts to move into major parts of each others' businesses, usually at the detriment of their customers or users.
Google, the geek world's undeserved, unquestioned darling for well over a decade, has made all of its core products worse by forcefully shoving Google+ into them. They're leveraging extreme success from some businesses (search, email, maps) to juice the numbers of one that's faring poorly against its competitors (Google+). Sound familiar?
Apple's Maps is still worse and has fewer features than Google Maps, which was previously integrated better into the iPhone and didn't enable as much Google tracking creepiness. Not anymore. (Although I think the fault of this is shared between Apple and Google.) Many of Apple's other applications and services have suffered as well as they've spread themselves too thinly and competed on more fronts.
The battle between Twitter and Facebook has made both products worse and caused weird restrictions to users on both sides, such as the walls both companies have installed between Twitter and Instagram. Twitter is now ultra-paranoid, defensive, aggressive, and full of annoying ads. Facebook's core product is a mess as it continually tries (and fails) to capture the usage and style of Twitter, while annoying people more and more to keep its ads effective. (At least Facebook is consistent: they've always been getting worse.)
Amazon making its retail business worse to prop up another part of its ecosystem shouldn't be a surprise to anyone. Amazon doesn't want you to be only a retail customer anymore, and they'll keep making it harder to be.
They want to lock everyone into everything. Just like everyone else. And we're all worse off for it.