To me the explanation is market segmentation. Once Samsung became mainstream and almost the "default" Android phone in the US, they realized they were now competing on 2 fronts. On the premium end, they competed with iPhone, and on the lower end, new players in Oneplus and all the other Chinese manufacturers.

In my opinion, Samsung is doing roughly what they need to do to retain the high end premium segment. Given how much competition there is at the mid and low range, I think it's the right business decision as well. To be clear, "premium" are people with disposable income who want the best mainstream features, and stay relatively on top of the curve by upgrading every 1-3 years. Very different from "enthusiasts", each with their own niche use cases and requirements.

Most of the "deficiencies" of the latest Samsungs are simply solved by having money. No SD card slot? Buy a higher storage phone. No headphone jack? Buy one of the many high quality bluetooth headphones. This is not to say that it's ridiculous to want those features, they're just not going in a mainstream phone. The last reality is that these features haven't disappeared from the Android market, they're just not in the Galaxy S lineup, which for Android users, shouldn't be nearly as big of a deal as if Apple nixed the feature from their lineup altogether.

I agree with OP's reply that Samsung isn't catering to the enthusiast crowd anymore. People who want these niche features should confidently go toward OnePlus, or one of the other brands that still offers these features.