Before the GFC the index was 1550, and after it was ~760 (~50% drop)
At the peak of the dotcom boom it was generally under 1500. In 2002 after that crash it was 800. (45% drop)
The recent peak was ~3380. A 50% drop is ~1700, which is almost 30% below where we ended today.
The question to ask yourself is how does this collapse compare to the dotcom collapse and the GFC?
The dotcom collapse affected internet businesses and was contagious (sorry) from there.
The GFC start with banks and finance and was contagious from there.
This starts with pretty much every industry, and has immediate impacts on supply and demand. It's going to be very ugly.