Two comments:
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I’m using my left earbud like this (i.e. “open”) for at least six months, and no visible or audible sign of it working worse than the intact right one. I’ve made it a habit to strongly blow into it once or twice after each use, before putting the earbuds back into the case (I never keep them out of the case if not in use).
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Apparently @paloma has found fitting replacement mesh (grid).