Putting everything in Bitcoin is MUCH safer than putting some of it in altcoins. You can make money with altcoin trading but none of them comes nowhere near the store of value aspect of bitcoin. When it comes to long term investment bitcoin is the safest crypto option by a long stretch. Of course one cryptocurrency doesn't mean one wallet, you should it spread in many wallets (only open source wallets of course like Armory, Electrum, HODLER or running your own node with Bitcoin Qt), securing backups and so on.