Like you, i'm glad that David explained how it works (and why), but i'm totally ok with your proposition.
I think it's risky to loose existing translations. And i suppose that, most of the time, plugins maintainer do small corrections (typos...), and don't reuse the same string identifier for a completly different string.
thank you for adding your feedback and for confirming my thoughts.
I would now create a MDLSITE issue to let David think about changing this behaviour for plugins, but before that I am wondering if plugin language strings are handled differently from core language strings during auto-propagation. Normally, I don't translate core language strings myself after they have been auto-propagated, so I simply don't know.
Thus, dear core language pack maintainers, can anyone of you tell if this also happens for core strings? If not, this would be another argument to have the behaviour changed for plugins.
Just to add that, sometimes, new strings just correct some typos, and these errors didn't exist in translation.