dotcom bubble may have been a thing, but the underlying tech was valid and created the big techs out of thin air. Admittedly after much pain & flushing out the pets.coms of the world
If we're at another moment like that do you really want to be sitting on the sidelines?
The trouble with a crash is that it happens fast, and you are likely not going to be able to get out fast enough. There is also the psychology involved - say you lose 20% in one day, then do you sell, or wait for a bounce back before selling? What if there is no bounce, and it goes down another 20% the next day, then what do you do?
Nowadays there are "circuit breakers" that will stop the market if it is crashing, so maybe you hear about the start of the crash an hour late, by which time the trading has been halted. What do you do now - put in a market sell order? If every one is trying to bail out at the same time, then the market will reopen at a much lower price (matching buy & sell orders), and so it goes ...
However, say you got onboard late and were "only" up 300% before the crash, then post-crash you'd have been left with 0.2 x 4.0 = 0.8 (80%), so you'd have ended up having lost 20% of your original investment, and of course the later you got onboard the worst off you'd have been.
And google gained how much in subsequent years?
Not disputing it was ugly & bloody, but rather saying the underlying tech was valid and the companies that built on it in a sound way crushed it.
Tricky bit is telling which of the current crop are going to be the survivors
Diversifing is smarter than timing the market.