Problem is, we live in the present. We cannot invest today's paycheck 3 years ago, we can only decide to invest it now or to wait for some time in the future.

It's even worse than just paycheck coming in "now" to invest "now". We generally have only about 30 or 40 years to invest the bulk of our savings. It's generally only over age 30, and for most over age 40, when there is already a substantial amount of savings that needs to be invested, so really you only have 30 years of investing (followed by drawing down those investments in retirement). So, if you stay out of the market until "AAPL reaches 100" (or whatever arbitrary measure of value you are using, you might be out for 5 or 10 or 15 years, a HUGE portion of your primary investing lifetime.

Now if you're going to compare S&P500 to T-bills as we have been doing here, there's yet another thing to take into account. If there are ever such bad economic conditions that cause the S&P500 to have such a dismal 5 years, there is a VERY strong possibility that the Fed would lower rates dramatically during that period. Perhaps they would even lower them to near-zero, it wouldn't be unprecedented [anymore]. So even if the S&P500 has a dismal 5 years and earns only 5 or 6 percent, it is also quite possible that those T-bills don't protect you from that and also earn in the low single digits over that period of time.