In my opinion, investing in just one coin is not profitable, but when we have limited capital that can be used to invest, we can buy coins that have long-term potential and don't care how many coins you invest.
That's what we two were agreed on, I think everyone in crypto space will agree on this fact that investing in one coin is not profitable, besides the factor of how much funds you have, is also considerable, because if you have lesser funds then diversifying it won't get you more profit so its better to invest it in some old, and trusted coin, maybe more then one like two or three, that would be enough I suppose.
For example, if we have 100$ then investing 20$ in 5 different coins, of $10 in 10 different coins, of $50 in two different coins, would be more better. It also depends on the risk that a person wants to take, like if a person wants to take a risk with half of his money ($50), so he should invest in highly volatile coins, that could either give him more or lesser. But the remaining 50$ should be invested in such a way that they could at least generate you another $50 + the profit. Because That's how you manage the risk. Play safe with half an play risk with the other half. What do you guys think?