If you want to protect yourself from negative interest rates, the best way is to invest into long enough government bonds.
The value of a bond increases as interest rates decrease, and decreases as interest rates increase.
Essentially, by buying long government bonds, you are "fixing" the interest rate you are getting for the bond part of your portfolio. Then, if the rates decrease, the value of your bonds increases so that the effective rate you're getting based on the new increased value is the market rate.
But, that "fixing" of interest rate has a cost. If the interest rates increase, you will still get the old rate, meaning your assets are worth less after the interest rate bump.
But I wouldn't consider negative rates necessarily bad for the investor. If you're young, you can take a small loan for investing into stocks, should the bank be willing to give a loan. I did that; I loaned 23% of my annual gross income, at a true rate of 1.3% (including all fees). Ok, the rate is not negative, but it's significantly below the dividend yield which is probably around 4% for the local stock market currently where I live. 10 years ago, the interest rates were so high that I wouldn't have invested borrowed money. (And in any case, I'm not going to let my equity ratio fall below 40%, so nearly half of the money I invest is my own.)
In the US market, situation is different from Europe. The interest rates are higher, above 2%, and the S&P 500 dividend yield is below 2%. So, borrowing money in the US and investng it into US stocks doesn't make much sense unless you are doing it for the very long term (let's say for 40 years), and willing to take the risk.
For someone old and going to retire soon (or already retired), the situation of negative rates can be bad, because they should keep most of their money in bonds, not stocks, due to expecting to spend the money soon. But, as a person who's not going to retire soon, I do enjoy the negative interest rate environment.