I think if you look closely, the switch has been ongoing for decades (yes
maybe even the century plus that JD indicates) and will eventually just be.
We already do most of our liquid measure (with the exception of the gallon)
through metric (soda and liquor leading the way), and I think every
foodstuff is measured in grams and/or ml even if they are also marked in
ounces. The estimated cost, back in the 70s, for a complete switch over was
immense because of all the retooling (including the tools) that was needed.
But since then several things have happened. First and foremost the
automobile market was taken over by the Japanese--with metrically engineered
cars. Even many of the GM/FORD/CHRYSLER models are essentially imported
vehicles. I'm not sure how many '1/4-20' screws still go into an American
car, but it might well be none, and will eventually be none. When
factories, those few left that actually produce fully marketable consumer
goods, start up or retool, more than likely the production machinery ends up
imported--and I suspect made with metric parts. So slowly but steadily, I
am sure the industrial world of the U.S. is becoming metric. Certainly if
any company wants to export their goods, they will be sure they can be
serviced with metric tools.
The remaining major hangups:
1) The gallon: Which will probably remain, at least for fuel sales, but
one day may morph into 4 liters. The fact that the liter and the quart are
so close, is probably why the food/beverage industry was able to change so
easily. Back when the first oil crisis hit, I remember some stations tried
to sell gasoline by the liter (so the unit price would remain about the same
as it had been). That experiment did not last long. When driving in
Canada, I unfortunately can do the math all too well and realize what the
price per gallon really is.
2) The mile: Ingrained in the culture, I think. The 4-minute mile and all
that...and the conversion to kilometers is not simple (OK 100 km to 61
miles--say approximately 60 seems simple enough to us--but we can do math,
the common folk (including far too many of my students) cannot! We may go
back to dual signage, but I think we'll have the mile for a long time.
3) Fahrenheit temperature: Again, ingrained in the culture and not simple
to convert. We do generally report weather in both scales, but most people
ignore the Celsius temps (usually misnamed Centigrade).
One thing to remember is that this is a nation of over 300 million people.
Changing the habits and culture of that many is difficult and can lead to
the kind of divides we see in national politics. ;-)