So without giving financial advice, I'd say it's more complicated than throwing it in an index fund if your time-frame is a decade.
We aren't talking about losing sleep over volatility here. We are talking about a situation where someone specifically wants to hit $1M in a decade.
For example, I put several thousand in a Vanguard TR Fund in January of last year. It's still down 2% overall. Now on a 30 year horizon, no big deal. But for a decade that will significantly impact the compounding returns. Stocks are very volatile and may not be real desirable for a 10 year time frame.
Even a ten year average returns is typically going to require more -- a cycle longer than twenty years. Financial services are sold: caveat emptor.
Let's say a 7% annual compound interest over 10 years (which I think, is debatable). That puts you at $5,800/mo. I think my argument still stands?