If you are only putting your money into total market index funds, then how would you know what you are buying because you are buying everything, and you definitely can't know or understand everything. Hence you are betting that America will be more prosperous in 15 - 20 years when you buy at today's prices. Keep buying every other week with your paycheck and expect a 7% yield on balance. However, the potential yield on your SPY purchase today SUX and could easily go nowhere over the next 10 years. While dividends may be a better yield than bonds right now (and that is just crazy), the risk of downside is quite high.

Full disclosure, I have a couple things that I'm buying for the long-term today. But these things are priced at a steep discount to what I THINK their long term value is. I'm hoarding lots of cash. This time last year, actually more like at the beginning of the year last year and at Christmas of 2018, there were some really easy discounts to find. Some.

Real investors don't time the market, they wait for the market to serve them Apple at an 8 P/E. They wait for the market to freak out about E-Coli and a really great, cash-generating burrito business.