A Bitcoin sidechain is what it is-a sidechain.Bitcoin cannot "metamorphosis" into any altcoin,because the Bitcoin Core blockchain cannot be changed.
I remember that many people in the BTC community were talking about the Atomic swap 2 or 3 years ago.
Nobody is talking about Atomic swap now.I guess that it will be pretty difficult for such sidechain projects to get mass recognition and mass adoption.The vast majority of the crypto users don't trust sidechains.
All the legit altcoins like ETH cannot be dethroned by such competition.