Bitcoin is volatile and its movement is not that stable, it can either go up and down so you need to invest also in other cryptocurrencies.
Well it seems you're not following the market then if your advice to invest in altcoins revolves around the reasons you gave and I qouted above. Altcoins are even more volatile than Bitcoin so if you're trying to minimize your risk, either make use of stablecoin or go invest in other assets available like stocks or previous metals.
Investing in altcoins should be for the technology, as they have quite a lot to offer to the industry. The Ethereum blockchain as an example has brought in quite a number of development that if it wasn't to be in existence won't have existed. Forget about the other so called Ethereum killers because without ethereum the possibility of them coming into existence is minimum.
I love altcoins, they're quite interesting if you take your time to study them but never invest in them just because you think going 100% in bitcoin is risky.