Perhaps one day Bitcoin will become an altcoin, if BCH overtakes it. Although that is unlikely. In essence, any blockchain that is not BTC is called an altcoin.
It just so happened historically, everyone calls altcoins all coins except bitcoin. It seems to me that it is irrational and somehow "racist". Each coin has its own name, which it must be called. And I think that someday bitcoin will go to the background and its place will be taken by other coins.