There is no guarantee of anything. Bitcoin or ethereum is not safe investment as well. No one is taking guarantee of both coins. They are also categorized as a high risk investment so it may value zero so no one can take guarantee...
We also should not conclude that bitcoin is not safe.
The name of investment has its advantages and disadvantages, so everything we take has a risk, even if we invest in stocks or banks, there will still be risks.
See the change in the news of bitcoin now every year more and more people to own bitcoin and trust bitcoin or ethereum for their investment even every year the price of bitcoin is always changing and rising high.