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.
I believe that Ethereum can become a direct competitor to Bitcoin and be able to get ahead of it in the cryptocurrency market. Of course, I understand that Bitcoin is a pioneer, but cryptocurrency users in the future can evaluate Bitcoin as a rarity, and this may be its value. And Ethereum has every chance to become a leader in the cryptocurrency market, since the speed of transactions, their value, as well as security at a very high level.