That's a lot of serious caveats at the end there. If you don't have the financial resources to create a "massive emergency fund" then the point is moot.

If you do have the resources, then my answer to your question in the title is, "You won't know you need a massive emergency fund until you need it (the name kinda gives it away). If you do need it and you don't have one, you're fucked. Maybe for the rest of your life. If you have the fund but don't ever need the fund, you lose some interest you could have earned. Which is the better plan: Risk possibly being fucked for life or forego some potential interest?"