Technically, Prime is not an entity, it is a subscription. It can't be "greedy".
More importantly, Amazon does not, and can not, control the digital content it has. That power is granted only to the publisher, and only for a short period of time. Typically contracts are on 1-2 year cycles. Content will come and go, sometimes piecemeal, as those contracts end and are renegotiated. That is how copyright law works. And for the record, it is not any different with Apple or Netflix or Hulu or anyone else. In fact Netflix is as notorious for changes in content as Amazon is. And it is getting worse as more and more networks create their own subscription programs and remove their content from Amazon/Netflix/Hulu/etc.
Also, you mentioned Apple. In case you missed it, their Apple TV+ service is going to be exactly the same as Amazon's. A bunch of general content culled from what they sell (as allowed by the copyright holders), some content they produce, and a whole lot of extra charge separate subscription channels (HBO, CBS, EPIX, etc) that they are calling Apple Channels. Given what your complaint is, it is odd you would mention them as they are exactly the same.
I did look at the specific example you gave. I just did a search and looked through the Masterpiece content. I am still seeing most of it as included with Prime.
Usual complaint from someone who finds their favorite show has dropped off the free list and is horrified to find out the people who make that stuff actually expect to make money doing so rather than giving it away for almost free forever.
"It seems lately that Prime Content is disappearing. " would seem to indicate the OP never read how this works as that's the normal state of things: They come, they go, sometimes they come back, some things will never be "free".
Entire swaths of stuff are vanishing because the rights owners are going into business for themselves.
People also forget Prime is a shipping upgrade with a lot of extra perks. Netflix is a dedicated streaming service but, if the OP does due diligence, he or she will find that Netflix's DVD side has about ten tomes more offerings and is a separate subscription and that streaming stuff on *every* service out there comes and goes and with more and more production companies starting their own services, even the ones he favors are going to disappoint him or her before long or require hefty extra subscriptions.
Disney here, Warner there, others elsewhere, it's going to become very fragmented.
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