Please let an engineer living in Scandinavia comment.
Timber!! Wood material measurements were the last to be accepted converted here. In practice it is easier to handle and understand the metric system once you are used to it. It will also give a more accurate description of tolerences, often given by (say) +/- 1 mm thickness of a plank from different sawmills.
Valves and piping: Once, at an international exhibition in Germany, I asked a Chinese representative of a Chinese valve company, competing with mine, if his factory could supply ANSI and European flanges. This was some years ago, and Offshore North Sea used (at that time) mostly ANSI. The answer: 'We can supply both, but we wish mainly to supply European. We see the largest potential here as they have a smaller market now, and we want to grow and compete. (No comment!)
It is not seen as a large problem in Europe to connect ANSI and European dimensions by suitable connecting pieces and/or conerting flanges.
On driving: I have been driving on the left (wrong) side in Sweden, UK and Australia. The problem is to use a system where you have learned instinctively to turn to nearest ditch to avoid accidents. If you converse from right to left, or opposite, this will then lead to front collisions. Also driving a car with the wheel 'on the wrong side' is aproblem.
And lastly, on different feet: To me this seems to be a 'microscopic problem' compared to the other topics discussed.