Years ago, if anyone had suggested to me that an American could assimilate themself, to the metric system....I would have had doubts.
I was introduced metric in high school, and it was generally a wasted exercise to get everyone into using various numbers to convert. It had no real practical application.
After a two-year Germany tour in 1979 ended in my Air Force years....I would have suggested that it's complicated to fit into the metric system and you have to continually convert yourself from one system to another.
At some point in mid-1990s....after coming back to Germany....I suddenly started thinking at least in distance and liquids...in metric, with ease. I could envision a .3-liter of beer, or a half-liter beer, or a complete one-liter stein of beer. I could envision filling a gas tank up with forty liters and how far it'd carry the car. I got to the point where I could look at distance and tell you this was 200 meters, or how long it'd take to drive 200 kilometers. There was no more conversion for me....it was simply observing things and accepting them.
Issues? Well....yeah....I've come to realize you do need a metric socket wrench set. You can't make an English socket set work in every single case.
Yeah, I still have trouble with weight in the metric system....mostly because I rarely have to think over this topic.
So the truth here is that the only way you can fully appreciate the metric system....is if you live in it. Otherwise, you will never get around to the system and accept it.
The world to eventually accept metric.....no. It just isn't going to happen. There's always going to be this split. That's the simple truth.