Diversification is only discussed because you can't hold any one coin. Some altcoins have much more potential than bitcoin. Right now, only bitcoin and a few altcoins are up. But when the bulls really come back to the market, the other coins will also show good growth.
I corroborate your thought but In this context you cannot find out those potential coins. Moreover there will be a risk factors. But if one keeps increasing his investment in Bitcoin gradually then in the long run he can grow his portfolio. Investing in Bitcoin we can't give 100% certainty but we can be very confident that it will find a new ATH. But when I invest in any altcoins no one knows if it will ever reach its previous ATH or not. Diversification of investments is of course important but it can be through buying any other asset like real estate or gold etc besides Bitcoin. So that even if the profit is less, it will not priceless.