re:
The KitKat initiative is incredibly annoying, but many of the things you're talking about have work-a-rounds. Why can't you listen to your music on a plane, for instance? I do. The music can still be loaded to the card. It will play just fine with things like PowerAmp. I have about 100gb of mp3s loaded from my desktop at the moment onto my card. Works fine.
Samsung's native file manager will happily delete, rename files for you.
Why can't you read a book loaded from the card? I do. The files can be loaded to the card, the reader can be pointed to the card and will open them up. True, some apps like Kindle now will only recognize internal storage. If you're using Kindle, that's pretty much that. Kindle whispersync books will only go to internal storage--but really, how many of those do you have? It isn't that much space. Other eReaders like Cool Reader still recognize and use the card and happily open books from the card. (You can also use MY FILES to take a book to read with Kindle, "cut" from the card and drop it into the /Kindle directory in internal storage and be on your way reading. While that's an extra step, it's certainly easy enough to do.)
Any number of apps will play videos on the card. I haven't tested everything, but MX Player certainly will.
Document creation issues can be an issue, but hey--store to internal memory. Documents really aren't that big.
If you're downloading things automatically they may wind up in internal storage and you may have to manually move them. Yes, a hassle, but not impossible.
I still store all my media--videos, mp3s and non-Kindle ebooks---on my card. There is no problem accessing them. Renaming them..deleting them...depending on what you're trying to do and with what, that may be an issue. You can always shell out to My Files to do that as a last resort. Yes, that's incredibly annoying and a step that should not exist. I'm not disagreeing. But the card has not really been rendered useless.
L should also have native support for:
visual voicemail
wifi calling
Well, aren't those carrier-specific things?