[osigurdson]

Of course stock markets can go to zero but they probably will not. It is impossible to prepare for myriad near-zero-probability Armageddon events so don't waste time thinking about them. Strengthen the cognitive noise gate, focus on things within your locus of control.

[NovaDudely]

It is like theoretically you could just fall through the floor by just atomically shifting through it. It is possible but very unlikely before the heat death of the universe.

Stock markets are more likely than that but it is also something I wouldn't lose any sleep over.