5 kilometer = 5000 meter = 500 000 centimeter = 5000 000 millimeter. Now you do this for 5 miles (yard, feet, inches).
1/110 mile = 16 yards = 48 feet. Now you do this for an one-hundred-and-tenth of a kilometer.
Or...
1/3 pound = 1/2 mark = 6 shillings & eightpence = 80 pence. Now you do this for a euro. 1/3 of a euro = , ummm?
The point is, the traditional measures are built out of very highly divisible numbers, which is very useful in certain applications, for example surveying, bartering, baking, etc. Just like the metric measures are built out of powers of ten, which is very useful in certain other applications, like seamlessly moving between scales of magnitude in scientific work.
So freaking what. Use decimals and you can easily convert anything into everything. And you don’t have to rely on those few cases where you can exactly divide.
6.8m = 680cm = 0.0068km, etc.
Who writes out prices in fractions?
Its even worse with baking. Scaling up units from recipes is really hard if you have fractions of different units like tee spoon, table spoon and cup and when ounces can both mean a volume and a weight. (“fl. is not always specified”) No thank you.
Also your examples are really poor, when the base systems keep changing in the same dimensions (e.g. length) there is not benefit at all, just confusion or reluctance to convert to other units because it would be too awkward.
Being able to divide by 2, 3, 4, 5, 6, 8, 9, 10, 11, and 12 is not a "few cases", it's the chief part of the small integers into which stuff often needs to be divided evenly.
If you ever went to the market before decimalisation, you would meet many who write prices in fractions. Something would be priced at X pounds/crowns/marks per Y items/weights. Then, the cost of Z units/weights is XZ/Y. When the coins are highly divisible multiples of each other, you have a lot of choices to be able to pay the exact amount and don't have to bother with getting change back. Even better if we price by the dozens (Y = 12) instead of 10s: 12 has twice as many factors than 10.
I understand that this all requires a certain numeracy however. Thankfully nowadays we can just buy things using their credit cards on Amazon: makes mindless consumption much easier.