The options are to trade in Imperial measurements with the US, or the entire global economy in Metric.
The US can weather that difference given its size both physically and economically
The US doesn't use the Imperial system but it's own customary system. For example, the Imperial pint is about 568 mL, whereas the US customary pint is about 473 mL (liquid pint) or about 551 mL (dry pint). Both are 1/8th of a gallon as defined in each respective system.
(notice how I conveniently included the metric equivalent of both measurements?)
Distances are in meters if they are short, miles if they are long, Leeds will be 275 miles away from your gas station that would charge you by the liter so would be most other liquids unless they are milk or beer, then you’ll get a mixtures of metric pipes with inch threads and inch pipes with metric threads, when you buy a property the land would be demarcated in acres, build up area in square feet and priced in meters squared… and when you over eat because of all the stress of having to memorize 1000 different measurement units your doctor would tell you that you gotta drop a few stones…
I believe they are now technically defined in mm, but obviously it's still based on fractions of an inch. In my view, there aren't any truly compelling reasons to switch from BSP to something based on natural metric measures.
On the other hand, if we were to replace these pipe fittings with metric, we'd end up with a mixed system for realistically at least a century, if not a lot longer due to how long pipe systems last. The complexity (and also, in some cases, danger) of having to maintain both systems for so long would be incredible.
It's similar to how in flying, we still use feet/flight levels, and actually a few places have migrated from metric to the more conventional imperial system.
Imperial pipe fittings are pretty common across the entire world, in fact. I think most countries outside of UK/USA use BSP (often by a different name), but NPT is also sometimes used - and even BSP vs NPT is a huge pain.
So in reality there already is a mixed system which is often what causes a lot of headaches, what’s worse is that BSP is also a mixed standard it has both metric and imperial measurements for its standard threads however when you mix fittings that were machines using metric and pipes that were machined with imperial I’ve found that because of slight tolerance differences you often can get into a point where stacked tolerances becomes and issue.
It hasn't stopped the conversion of most other aviation measurements to metric. Runway dimensions, visibility and separation distances are in meters, temperatures are in Celsius, atmospheric pressure in hectopascals, fuel is in litres, mass is in kilograms. Frequencies are in Hertz. At least that's the case in Australia. Pretty sure Europe is similar.
In short after a lot of math it boils down to the fact that US threads are defined as threads per unit. and metric threads are defined as units per thread. "So what" you may say, "6 of one, half dozen of the other", but threads per unit ends up having a lot of common factors and can be cut easily and swiftly on a lathe with a threading dial, and metric threads can't. Here is a youtube video that explains it better than I can.
https://www.youtube.com/watch?v=zG9XYDCDwWY
If you ever see someone cutting threads on a lathe and they swiftly engage and disengage the power feed without stopping the lathe, they are cutting US threads on a US lathe. If they have to stop and reverse the lathe, they are using a metric lathe.
* Road distances are measured in miles.
* Drinks are ordered in pints.
* People measure height in feet/inches and weight in stones.