The best any coin can do is to displace another in terms of some arbitrary metrics. Ethereum was a pretender to the Bitcoin crown in market cap. I've actually seen altcoins whose single coin value was more than Bitcoin at one point. Don't know if it was ever really bought and sold on actual orders, but hey, that's one way to replace it.
But segments of Bitcoin use? Sure. I remember when Monero first spiked noticeably, in 2016 when supposedly darkmarkets adopted it. They didn't replace Bitcoin, but Monero took over any roles of anonymity that people mistakenly thought Bitcoin had.
Basically, altcoins are copies of Bitcoin one way or another. Some are a direct copy of BTC with the same source code and everything, while others mimic its properties/functionalities. If you take a deep look at the crypto industry, you'll notice that every altcoin is just an improved version of Bitcoin. As the pioneer cryptocurrency is the original project which started it all, altcoins will never be able to take its rightful place on the market. It's as simple as that. While this is true, I believe that a few rock-solid altcoins like Ethereum and Litecoin will be able to complement Bitcoin in many ways. In areas where Bitcoin cannot fulfill certain tasks, altcoins come into play. Given how Bitcoin is recognized by many as "digital gold", it's safe to say that altcoins are more like "digital cash" than anything else.
Of course, some altcoins are even much more valuable than Bitcoin is now (like 42-coin). But even so, they've been unable to surpass BTC's market cap for a very long time. If we were to classify the best altcoins that will live alongside Bitcoin, those will be Ethereum, Litecoin, and Monero. They not only have innovative features, but they're also rock-solid in development and innovation. Additionally, these altcoins have a good degree of usability within the mainstream world. For once, Monero was highly popular with darknet markets while Ethereum was the top cryptocurrency for ICO tokens. Those days seem to be over, as people have lost the excitement between these cryptocurrencies. It's now the day where both ETH and XMR are extremely cheaper than they were some time ago. Despite their many advances, Bitcoin is still the world's largest cryptocurrency by market cap.
Nonetheless, it's highly unlikely that altcoins will replace Bitcoin anytime soon. The pioneer cryptocurrency has grown big enough to a point where it's virtually unstoppable. Since the whole crypto market depends on Bitcoin as a reserve currency, the situation of replacing Bitcoin would turn out to be academic. Just my thoughts