As a holdout for English measurements who has to exist in a partly metric world, I have done a fair amount of thinking on this subject -- lubricated by beer, of course.
I think the basic problem with the metric system is that it lacks quickly identifiable units for everyday observations.
Like the kid who said the bullet missed him by an inch. What should he have said? The slug missed me by a centimeter? Two-and-a-half centimeters? Not only is centimeter harder to say than inch, the term doesn't mean anything in particular to many people.
Then there's the issue of tradition and the gallon. Gallons seem natural to old farts like me. Liters look like the proper measurement for a jar of peaches or my wife's dill pickles. (mmmm - her dill pickles are fabulous!)
So what's needed is a way to speak metric in short, recognizable words. I have no answers -- only dumb questions.
I suppose four liters could be called a gallon. It is -- almost. After a century or two, no one would know any different.
An inch, though. What could replace that handly little critter?
Seriously, I worked as a mechanic for years. Other than needing standard and metric sized wrenches, it was no big deal. Even metric engine sizes don't bother me. It's 61 cubic inches per liter. What could be simpler? My old 327 Chevy would never admit to being a 5.4 liter mill, but there's always a few cranks out there.
:O
jim