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You’re right, but we’re not talking about “at scale” here if I understood OP correctly. We’re talking about considering self hosting email for those who have the technical know-how to do so and obviously not on a rickety 2010’s desktop PC in your living room on consumer broadband as another commenter hinted at. Anything online “at scale” is always going to be harder than doing it on a small scale.
Imho it’s because docker does away with (abstracts?) many years of sane system administration principles (like managing logfile rotations) that you are used to when you deploy bare metal on a Debian box. It’s a brave new world.