What are your thoughts on the tension between transparency and privacy on public blockchains?

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Can you translate that for those of us who might not know what “zero knowledge by default means”? 

And also, what does that mean for this central tension during the next 10 years when the need for privacy and lack of trust in tech are at an all time high?

I suspect everything will be some sort of zero knowledge by default in 10 years. (big range, could be sooner)