It is very safe to invest in only bitcoin than going for altcoins. But be wise, bitcoin is speculative, but less volatile than altcoins which are riskier. Most altcoins have no utility and consider most of them to be gambling too.
Broadly speaking, Bitcoin is much better when compared to Altcoins, and the risks arising from Bitcoin are not too big compared to us taking Altcoins, even though both are places where they have risks, people will of course continue to use them, and actually if you want to invest in This altcoin also doesn't hurt as some of them are good.