There is one point some are missing about why specific non-SI units stick around...
Inertia.
Let me give you an example:
In the US all of the freeway systems are labeled in miles, the exits are numbered based on miles, the highway signs are in miles.
Well, except one. Interstate 19 is labeled in Metric. All the distance signs are in meters or km, the exits are numbered by km, and so on. Why? Because it was re-labeled during the push for metrification back in 1980, and no one has seen any reason to change it back to miles. There apparently were some plans to switch it back to miles, but the locals revolted because of the trauma caused by changing from km back to miles due to the exit numbers changing, etc.
There are so many things that are hard to change because of the trauma which is related to the changeover. Things as simple as switching the screws which hold on a switch cover plate means that all of a sudden, you have to stock and deal with both the new and old styles. And for things like this, it doesn't really matter. It could even be something totally oddball just for that application, and it still would be ok.
Take, for instance, the example of Interstate 19. One main reason why they haven't switched back is the cost of replacing all of the signs at once. If replacing the signs on 100k of Interstate is cost-prohibitive, imagine the cost of doing so along the 6.5 million kilometers of roads we have in the US. And, there really isn't any downside to staying metric as most people rarely ever cross into a metric country. The only difference in a modern automobile is the display units on the speedometer/odometer. Even that is moving toward being universal due to the trend to move away from a mechanical speedometer and odometer to a digital display.
So, if you look at the stuff that the US hasn't converted, almost all of it fits into this category: Stuff so deeply entrenched into the daily lives that it would be painful to change over. Or stuff that the units don't really matter for.