It actually depends...
I am here for long enough to see that a huge number of altcoin projects, although promising at start, now are dead, or dying, or at least far less profitable than Bitcoin.
...And I am not talking about scams.
So putting all the eggs into Bitcoin basket is for now a better choice than trying to earn a little from more different projects.
Of course, that is possible too as long as you are careful and leave those projects before they start going downside.
All in all, while what you are saying is correct on paper, altcoin scene has shown that the reality is different.
So diversifying has to be done with extreme caution. And in doubt, stay on the main project: Bitcoin.