"A strong soft shackle that can do the same job" How can your soft shackle do either end of this vang (see pic), and why would you want one here?
"Tow lines (dingy, inflatables, water ski, even other boats); they won't sink, dragging the line down where props are," Splicing is a very useful skill.
"2 went to keychains." Down to only one key; keep it in my wallet.
I certainly wouldn't use one on my jib sheets (7/8" double braid) as I'm positive they would fail at some point.
I'm sure they've been around a lot longer than 200 years, but that doesn't make them the best thing for the job. Canvas and flax sails have been around a long time too, but I doubt many of you are going back to them.
I'm not trying to be argumentative. In fact, 10 years ago I would have agreed with you on all of this, but I experimented and learned. I've never had a soft shackle failure, though I destroyed a few SS shackles in the same period. This stuff is seriously strong and tough. I'm just trolling for good ideas. I still use plenty of SS and aluminum hardware where I feel it is best, which frankly, is most places; no chafe, more compact in many cases, and lasts a very long time. Cost, taken over time, is a trivial factor. So is weight, in most applications.
Vang example. Why would anyone want to change what you have? I would not.
Splicing. I would assume that anyone that makes soft shackles can make an eyesplice with closed eyes. That is not the point. Often there is need for a releasable connection. I've seen these on commercial tow boats; they were large sizes and took serious strain.
Down to one key. Love it! I used that as an obvious example of "cute." Some folks make bags from old sails.
Coming off of sheet or failing. No, they won't. These are used on Volvo boats (generally dog bone variants, because they are faster to work with). You would think they could come off, but they do not. And they are MUCH safer on a flogging sail. There is no sensible argument for hardware on the clew of a headsail. These can be much stronger than DB sheets; a 5/16 Amsteel soft shackle tests 23,000 pounds; you'd be happy with 1/4" at 15,000 pounds.
As for your 200 year counter argument, there is an obvious hole; we still use sails, but with modern materials; polyester made the difference. In this case is is HMPE that cause the rebirth of soft shackles, though they can be very good in other modern materials. So no, we are not going backwards, we are moving forwards, asking what modern materials can do for us.