Nothing is guaranteed if things go totally to hell, of course. (And I did make the qualification of not being pegged to inflation.) But unless we get to a very bad place, I think that assuming 5% on these sorts of investments is a pretty safe bet.
(ADDED: But pick your own number. 3-4% probably is closer to the really very safe modulo inflation given instruments like CDs in that range. But you're reinforcing my basic point. $2M is not really the basis for a huge annual income in perpetuity or even over a multi-decade timescale if you don't have other income streams to add to that mix.)
Past performance does not promise future results, and the world could end tomorrow, but it's not a bad idea to plan that you'll get an average of 5%+ on market investment.
(I'll note that you're responding to someone saying "pretty much guaranteed" and dropped that qualifier in your question).
ADDED: Yes, you're "rich" but not jet-setting in business class at resort hotels, multi-million dollar coastal city condo, eat at top restaurants on a regular basis etc. rich. Which is actually just fine for a lot of people who have no particular interest in those things.