Thus, for most people the best way to invest is dollar cost averaging into spy.

For a long term "one click" strategy I think one can improve on that while keeping all the simplicity: if one is saving regularly, dollar cost average into a non-cap-weight index fund such as RSP.

It has a tiny fraction of the company specific risk of SPY. Top 3 positions in SPY make up 19.9% of the total portfolio at a weighted average P/E of 42.8, versus 0.6% in RSP which has an overall P/E of 19.0. Those biggies deserve a premium valuation level...but I doubt it's that much.

And equal weight has better returns over the long run and in most (but certainly not all) five year periods. You're never overweight the current big bubble stocks so you'll lag SPY in the occasional gigacap upswings like the last 1990s and lately, but generally do better the rest of the time.

And it's still just one purchase.

That falls into the category of "do what I say, not what I do". I have in effect built my own equal weight index fund, picking ~75 big stocks with what MI tells me have slightly better than average economic characteristics. I've only been running it with real money since March, but it's doing OK. I have a drag from a small cash allocation and from high taxes on dividends, but the net portfolio balance after tax is beating my benchmark (RSP) so far by 1.70%/year rate. The advantage would be 2.10%/year rate without counting the dividend taxes. Very early days, but consistent with the small advantage I'd expect from the backtests.

Perhaps the best argument *against* buying an index fund is rarely mentioned:
If there are any companies at all that you would not invest in because it is against your ethics, or even just a matter of taste, or of risk, then you can't play the index game. There are some firms that I find odious, and I'm not willing to take the risk inherent in companies domiciled in certain jurisdictions, so I have a short black list I use to filter my quant picks before looking at any other criteria.

Jim