My view on waiting for crash vs staying vested:
Tempting to wait for a 30-40% crash/significant correction but in the few months/ years waiting time, the market may gain more than the amount dropped in the crash. Plus you may be anchoring to the point where you first saw the index (at maybe 1500). Say the market rises to 2000/3000/4000 and afterwards your 'crash' towards 1600 comes. If the market don't 'crash' below 1500, then don't buy and later continue bull run no. 2 from 1600 how? When exactly do you decide to bottom fish?
On the other hand, people who are vested need to be able to tahan the correction and not cut loss at the worst possible time.
Anyway, I believe that looking at a long term horizon, even if the crash is tomorrow (worst case scenario), all-in today still has a very high chance of making you a profit 20 years later if you close your eyes and not cut losses at the bottom. That may not beat the market or even inflation but you probably beat holding cash for 20 years or net losing money due to many bad timed investments.
No right nor wrong answer, since it also depends on your objectives.
While I think staying vested is the better option compared to waiting for crash, I think it's still possible to time the market somewhat. Maybe on average (dunno how researchers calculate the average) people lose to market returns, but there are many who beat the market over the long term.
About market timing, from personal experience I learnt that:
1) No one can perfectly buy at the bottom and sell at the top
2) Won't be able to get the direction of the movement right 100%
3) Even if right direction 70% of the time, may still have sub-par returns compared to the market
4) Or if right 90% of the time in the past, maybe it's just luck (long 10 stocks in a bull market you probably guess the short term movements of 9 right?)
Still, it can be done with luck / skill / illegal stuff (insider info, connections, ++? etc). DCA + diversified portfolio + index investing won't turn 1M into 100M in 20 years' time. 1M into AAPL then TSLA would had... or in an alternate reality, one of the stock might have been GTAT