Protecting ones capital must start on how the individual handles the capital in the first place. Also, one must learn how to make the right decisions as to which coins to invest in order to earn profit. Diversifying ones portfolio reduces the risk of losing ones capital to a single project/coin.
That will be done if that person can learn about how to protect the capital. And yes, diversifying can be a solution to reduces the risk of losing money, but we still need to choose the right investment or coins to buy. That will not be easy if we don't learn something that will help us to choose the right coins, and besides that, there are so many coins that we can select as the next investment. When you can find a good project, and you invest with them, then in the future, you will make a big profit too so you can protect the capital plus you got the profit
Personally, I don't think that altcoins (which you implicitly refer to here) are a good diversification option
They may indeed be an invaluable tool for multiplying your wealth (read, for the purposes of speculation), especially if you enter a coin right before the pump (and exit before the dump), but recently most if not all altcoins have been performing worse than Bitcoin. So if you want to diversify your cryptocurrency investments, you should look elsewhere, not in the cryptocurrency market. On the other hand, if you want to protect you cryptoholdings from losing their dollar value, you can use shorts as a hedge (but then you expose yourself to the risks of using an exchange)
You can choose any altcoins for the investment, and I am sure you will found the potential altcoin
Diversification in cryptocurrency is one of the investment right now besides the other investment which we already know. If the altcoin is performing worst in this few years, I am sure that there will be a time for the altcoin to rising up again even if we need to wait for more years. But I see that some altcoin can moves to the higher price even if bitcoin price is down because that altcoin is not depends on the bitcoin moves.
I am sure most people who invest in cryptocurrency know about the risk, so they have a different strategy to prevent the losses that might happen to them. So as long as they can accept the risk, they know how to prevent the loss, they could still choose the cryptocurrency as the investment.