The US officially went metric in the late mid 1800s, and then about a hundred years later we officially adopted SI, which is essentially worldwide standardized metric. We're just using inches because too stubborn to realize that the other system is outdated, antiquated, and inefficient. We know better, but people don't wanna and you know...... How long ago did leadscrew handles go from having fractional inch divisions to just straight up base ten graduations?
Imagine your (milling machine, lathe, etc) having dials graduated at 128 of an inch, plus a vernier. And imagine what an improvement in somebody's life it was when they started seeing dials in 0.001 graduations. That's what the metric system does for every field of measure. It just makes life better. But you've got to think about it for a little bit to get used to it. So Americans (not America) refuse to go with it.
What I see in "metric takeover" is more like US manufacturers working it into the system. You started seeing things like cars with all inch standard designs, but a metric transmission. Or a major update (or flat out new) engine, it became a metric engine. Or a redesign of the front grille, and the fender "notches" for the corner lights had to change, so new tooling. Fasteners changed to metric. So it wasn't so much a date, as when it was convenient to stick it in there, because the tooling was being redone anyhow.
For equipment, it's the same thing, but it's a different world. Engines are legos, for sale anywhere to put in anything. And transmissions, and pumps, final drives, etc... They've been doing that for so long that the vast majority essentially had to be metric, so that they could be viably sold to the 15/16 of the world that doesn't use inch standards. I do find that in transitional years you are more likely to find a machine "almost all" inch, or "almost all" metric, but brands and models... There wasn't a concise time frame really. It was more about the global market for them, which came much later in the car world.
As far as a thread repair collection- What are you doing with it? Preemptive purchase, or are you fixing something in particular? Personally, I think it's money, time, and shop space ahead to order the ones you actually use, instead of trying to prep for everything that's possible. So the answer isn't where the world is going, it's "What's coming through your shop?" I'd probably, if it were me, grab a modest kit that puts your most used size right about in the middle, and grow from there. Very shortly going from "kits" to "bins", or your own larger compartment box to make your own kit, with the sizes you'll use. There's just WAY too many sizes, let alone places where one or the other insert isn't appropriate, to be "ready for everything". The "biased to inch" or "biased to metric" answer will sort it's self out in a way that fits what you do.
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