I agree with some of that.
Volatility doesn’t always pick up before the crash takes place in a simple sort of manner. For instance, before the 2000s crash, volatility had been elevated for years and before the 87 crash volatility had been sort of mean-reverting (low the summer before the crash, but much stronger in the weeks leading up to it).
From a Garch perspective, if you’re in a period of low volatility, then projected volatility should be reverting to the mean and be expected to increase. Similarly periods of high volatility would likely be followed by lower volatility.
Nevertheless, turning points are critically important in these type of strategies. It’s not just switching from low volatility to high volatility, which would cuase you to be in too risky a portfolio, but also high volatility to low volatility. In that case, you’re in a conservative portfolio, when you should be willing to get more aggressive. In other words, you would be missing out on the upside.