Hi, Madd.....I'll take a crack, but those were a lot of questions!
I've never had a card fail in a device. I go back a long way. (Recently, I was told by this site that I was one of the first 10 people who joined this site in 1999, when it was VisorCentral, and was still active!)
Documents don't take much space. I think over the years the people who want space tend to be media junkies. I don't need to back those things up--they are only copies from my hard drive or the cloud in the first place. They are on a card so I (a) have them available; and (b) don't have to stream.
I can only giggle at the people going "just give me a 64gb hard drive." That doesn't come close. When they come out with 256 internal memory, I'm in if it costs $10.
No, I am definitely not holding my breath, and the largest size I hear about is 128--which gives me less than I now have with my 128gb card and 32gb device. The truth is to get the space I want, it will probably cost an extra $100 to buy it again (and again) with each new phone purchase. Makes no sense. Really, aren't these phones expensive enough? There's a limit to everything. What Samsung is doing is increasing the costs of ownership with hidden gotchas and gimmes. It's like airlines first making you check bags and then charging you for checked bags.
Adapter: an adapter is one more thing to forget. And try putting the phone in your pocket with one.
It's easy to bend, break, you can't rest the phone down, etc. They weren't intended to be more or less permanent fixtures. They've been around a long time. There's nothing new there. It's fine for offloading or onloading stuff, but for permanent storage, things I use regularly--not a very convenient solution.
To answer your last q: Most of my storage is audio in nature, but I have on occasion wanted to watch some movies. It's not my preferred choice to do so on a 5.7 screen, but the times I've done it have been better than I thought it would be (and better than nothing). It just matters where you put the screen. One item you forgot, too, is modern HD video. That chews up space in a HURRY. If you're on a 3 week vacation, and you have a 64gb card that's largely filled, you may be seriously screwed if you have no headroom.