Im the other way around and think fractional math isn't used enough because it is so easy and useful. I think fractional maths biggest obstacle is everyone trying to avoid it and not learning it in contexts and methods where it excels.

That said, I still use tons of decimal math because sometimes it is more useful, but not always.

Even the existing-tool-holder is a weak excuse. My socket wrench set came with two near-identical sequences of wrenches, metric and imperial. I regularly the encounter metric variants in the US. Switching to metric, like the rest of the world, would make things simpler.

I keep trying to make myself think in terms of 32nds. For example think of a 9/16 wrench as "18", the 1/2 wrench is just "16", and so on. It's a slow process. I want to standardize on some color code for easy cross-brand identification of wrench sizes too, but I haven't come up with a compelling scheme.

The worst is the hardware. I inherited a full assortment of #2-#10 stainless SAE UNC hardware from a business move (already in nice parts drawers, too). It was pretty awesome for just having whatever I needed on hand to build things. But now as I maintain more and more things that are metric native, I've been building up the assortment of metric threads as well.

I suspect this is one of the real pain points of fabricators (plus taps/dies). And I'm guessing they're still still Imperial native due to existing tooling, making the conversions not clean (it's easy to convert 1/2 inch to 12.7mm and measure that, but it's not straightforward to convert 10mm to 0.3937 inches (25.2/64ths) and measure that.