The reason for SI is not because they were based on very precise definition: though indeed they are. It would have been possible to use equally precise definitions for Imperial units. The reason SI was adopted was because it had a totally consistent set of units that could readily be manipulated without the need for horrible constants as conversion factors.

Yes, some of these sayings will be lost, but I'm sure that equivalents exist in other countries that have used metric for years. Comments like 80 degsF - Phew whatta scorcher! will simply be replaced by the nearest equivalent round number: no-one will convert to the nearest precise equivalent because who actually notices whether it is 79.2 or 81.3?

However, one poster above spoke of leagues - they aren't in the Imperial system anyway. How many people know what they actually mean in distance? Not in the folk world - we can't even agree on how many leagues from Ushant to Scilly! Yet we all appreciate the saying, and so we will with many of the above when they no longer realte to contemporary practice, because we can get the meaning from context.

I'm told that German shoppers still buy some food by the pfund - which is a colloquial term for the half-kilo. Not a precise conversion - but who cares?