It isn't any slower than using celsius, especially with modern calculators and computers. Nobody cares about saving 3 seconds out of an hour of hand calculated

It isn't any slower than using celsius, especially with modern calculators and computers. Nobody cares about saving 3 seconds out of an hour of hand calculated mathematics when nobody does those kinds of calculations by hand anymore. And the same space probe loss could have happened just the same with pure SI units, the problem was somebody didn't put a unit label and somebody else didn't get clarification and made an assumption. If someone used decimeters without labeling and someone else read it as centimeters, you still failed.