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.
Why is that? (genuine question, thanks!)