1. This jumps off one of the lower points the Dow was at in the 1978-1982 range, when it was already about 65% off the 1965 high and had dropped below the 1974 low. Of course it's going to look really good from there. Spoiler: we're not at the same point in time in 2020. Jump off March, 2000 and see how those gains look compared to having sat in Treasuries or even a money market account.
2. Even with today's massive rally, everyone who's "systematically put money to work continuously" (read: dollar-cost averaging) in the market is still flat for the last 5 1/2 years. Not "flat after accounting for inflation," I mean "your total return over that period is 0.00%." As of yesterday's close, you were flat for nearly 7 years. If (when) we hit 2100, you'll be flat for 8 years. 2000? 8 1/2 years. 1250 (about where I expect we'll land)? 23 years of buy-and-hold gains will be wiped out. 25 points below that? It'll wipe out everything back to 1995. Inflation-adjusted? It's even worse.
Short-term timing? I agree, 99.99% of people shouldn't do that. Long-term "set it and forget it" a la Ron Pompeil, though? That's just as dumb, IMO. Gains are paper only until you sell, and way too many people won't do that because they fear losing more gains. Buy-and-hold is great as long as stocks go up forever. When they don't, buy-and-hold quickly turns into losses because your cost basis has increased over time. Never be afraid to take gains, don't just put blind faith in "everything will always go back up" because like irrational behavior on the way up, the market can stay irrational longer on the way down than you want to believe is possible.