Its your choice mate. It depends on how you wanna take profit and how long you want to hold your investment. If you are planning for long term then Bitcoin is enough. Cause it just pumped 4x or more within a year! Besides if you are looking for short term profit then you don't have any option without buying altcoins cause only altcoins will give you more profit within short time. So choose your coins according to your wants. But holding top altcoins along with Bitcoin is wise decision
If we are the first buyers during the preesale, we are the champions and what you just said is true, right, But if we enter when the hype is going on, for example when the price is going up and we predict it will go up again, that's when our desires start to disappear little by little. especially if there is a potential for a BTC correction. That's where the roller coaster was created by the whales by selling the coins little by little, not all at once.