I don't think Altcoin will ever replace Bitcoin one day! Bitcoin is not just a cryptocurrency, it is a means of payment, a belief of all traders around the world. The popularity of BTC and its mystery has not been surpassed by any currency.
This is been discussed before as to why altcoin cannot be replaced bitcoin. The features of bitcoin and it's popularity that people use it through these years. Many crypto trying to surpass bitcoin but until now this cannot be happen. As we look into bitcoin is the most in demand and top leading crypto currency that even altcoin can't replace it.
until now bitcoin is still a popular coin, while altcoin is still below altcoin for popularity. the reason is because the use of bitcoin is more widely used than altcoin, and many companies and payment systems use bitcoin in several countries