What will happen if the SI units are not used?

Answer 1

There are two related issues. Stopping the use of standard units will cause total chaos. You cannot have trade if the two parties do not know what quantities they are dealing with. And without trade most countries' ecomonies would collapse. Also, you would have distasters like the Mars Climate Orbitter - a lump of scrap metal that NASA sent, at enormous expense, to Mars - useless because they mixed up SI and non-SI units.

Second, one of the main advantage of the SI system, and the reason that it has caught on across the civilised world, is that moving from larger to smaller units (and conversely) is all done using factors and multiples of 10. Since we count in the base of ten, these conversions are relatively simple. Just knowing a few prefices is enough whether you are working with mass (weight), length, volume, time, electric charge or whatever. The Imperial system was an absolute jungle of multiples of 2, 3, 4, 6, 8, 12, 14, 16 and so on. This requires familiarity with so many multiplicatin tables.