If you would, what type of time period do you look at for the "crash"? Obviously traders are in and out on a daily or weekly basis.

No my research is based on the entire market data, so there is no short or long term, the correlation persists up until the present.

Now it may change locally, as we see decreasing between BTC & ETH, but it's still too strong to ignore it.

Arent they simply better off weathering the storm when holding BTC or ETH? Playing the move from one to the other can protect the downside yet could also cause someone to miss the bottom and a quick run up.

That depends. I'd say no. Since in every single correction, there is an opportunity to make 20-30% profit basically for nothing.

Of course if somebody stores like 5-10 million $ of BTC on a hardware wallet then its impractical or risky to move all of it into and exchange. But they could potentially make 20-30% if they would.

So it's really up to the risk appetite of the user, and how careful they are handling their money. On the exchange though, it's unfortunately not up to them, since they have to trust a 3rd party.