I have recently come up with some articles suggesting that the recent drop in the Vietnamese stock market is caused by the downfall of Bitcoin.
It might be true in the short run, but it is dangerous to use a single period of events/data to conclude that there exists a causal relationship between the events in the mid or long run. It is also wise to question those articles that rely wholly on correlation, as correlation itself does not prove causality!
If you take a look at the rolling correlation between Bitcoin, S&P 500 (SPX), VIX volatility index (VIX) & Gold (GLD) below, you will note that the correlation is actually quite weak. It is roughly within the range of 0.25 to -0.25. It shows that Bitcoin is still relatively unaffected by the volatility of other asset classes, though this line of argument is subject to argument as there is a potential spillover effects from oil and other comodities to Bitcoin (See: https://econpapers.repec.org/paper/prewpaper/201812.htm)
As an alternative investment investor, I do think that cryptos have a place in developing a diversified portfolio. However, its diversification effects in different economic environment are still subject to debate. While the market is still testing out the diversification effects, it is wise to have a small portion of one's portfolio in cryptos so that you can benefit from the blockchain revolution while not risking too much of your own money.
I cannot give you an exact number for the best fit of cryptos' share in your portfolio, as it depends on your risk awareness and ability to bear the risk. Having said that, I would oppose you to put all your money in bank savings in a low or negative interest ate environment.
Source: Sifrdata