You're right, why bitcoin can't be replaced with new altcoins like ethereum for example. why until now bitcoin is still the king of the crypto world ? is it possible because bitcoin have many users and is more trusted for an advantage in cryptocurrency?
You have answered your question with one of the most common reason why Bitcoin sits atop of all crypto.
Bitcoin is the mother of all crypto currency, and It took several years for it to be recognized and used as a currency for payment. There are already millions of Bitcoin in the circulation before the 2nd crypto currency was created. So, if you come to think of it why people choose to have Bitcoin in their bags rather any other altcoins, it is because Bitcoin has been used for payment long before all the alts in the market today, and thats what you call the mainstream acceptance.
The only way that I see an altcoin replacing bitcoin is when there is a massive prohibition on bitcoin SPECIFICALLY or if another altcoin surfaces and improves all the areas that bitcoin failed to meet. But I seriously doubt that this will happen anytime soon considering the number of users and the price of bitcoin in the market.