The publishers of some of the best-liked science fiction and fantasy magazines found out about this from Amazon
and have started tweeting about it. The SF/F magazine field has had some hits, and this is going to be like a giant hammer.
I don't like reading most magazines in digital form -- but SF/F and mystery magazines were a great fit. Many of them are digest-sized magazines that can be read even on an eInk device.
Analog; Isaac Asimov's Science Fiction and Fantasy; The Magazine of Fantasy and Science Fiction; Clarkesworld; Uncanny; and others get a lot of subscribers through Amazon Kindle. (This probably applies to Ellery Queen's Mystery Magazine and Alfred Hitchcock's Mystery Magazine as well.)
Now, it looks as if the publishers will be forced to sell the magazines through Kindle Unlimited.
But not all the subscribers are on KU, and I don't think anybody is going to pay for KU just to get the magazines they are already willing to buy. So some magazines may not survive.
There are alternative ways to buy them, such as
Weightless Books, Nook, and others. And many magazines are on Patreon. But of course, not everyone knows about them. Not everyone knows how to sideload, how to email something to their Kindle, etc.