This description has it backwards. When an altcoin is profitable, many people mine it, which sends the difficulty of mining skyrocketing. Remember that for most coins, the daily production is fixed. So when more people mine, it just gets harder to mine it. This means that the profit goes down, but the price of the coin is not necessarily affected.

It's also not really true that Bitcoin does everything that altcoins do. It's just that most of what altcoins try doesn't seem to matter much in the market, with obvious exceptions for stronger anonymity (xmr, zec) or smart contracts (eth).