"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.