As an investor, all I care is a strategy that earns me money. Frankly I'm not concerned what academically might happen. We all know that probably of accident while driving a car is greater than zero. You can worry all day and looking for opportunities to jump out of the car. The rest of the world will just keep moving on.
The market is extremely resilience. This kind of unnecessary worries is exactly how people got killed in this market.
I just can't see anything on the horizon that could derail the market. Just like Sydney property market, as crazy as it might look, the structure of the market means there's only one direction the market could go, and that is up.
Don't bother sell, but rather focus energy on how to increase your gearing. That's how to outperform the general market. The only rational thing to do in low rate environment is borrow to invest.
The question shouldn't be when to sell, but how to buy more.