I've wondered about this, and, well, a sac in use is not wholly submerged in ink - one side of it is exposed to the air all the time, and some of the inside depending on ink level, sloshing about (technical term), and so forth. Could this make a difference?
I worry we may have unintentionally biased information to work with simply because the first, and I suspect, only question a repair person asks when faced with a problematic sac failure is "What ink did you use?" Does anyone ever ask "What pen flush did you use?" or "What's the chemical analysis of the tap water in your area?" Do we know how ink changing (effectively mixing, because you never get a pen 100% clean) or fallow periods between inkings might have an effect? It seems to me a lot of very definite conclusions are drawn on anecdotal evidence and entirely natural and understandable human bias. So while I'm certainly not wedded to defending every ink from criticism, I do find it less than satisfactory to condemn them with the information we currently have.
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