Bitcoin is a genuine item,
and Altcoin is a sub generation of the genuine item which is improved by developer/people.
in brief it's like this :
Bitcoin created for them to send their money around the world without restriction and anonymity.
and altcoin created based on the Bitcoin itself with new innovation and discovery from other developers.
for example we can see Ethereum,
it's created based on Bitcoin and then they made a new discovery in token system,gas system and else.
but the most important this is,no matter what is it,it will never replace the genuine.
as we can see a lot of people using an altcoin as a tool for them to get more Bitcoin,because it will never replace it no matter what.