The big challenge for altcoins will be when pegged sidechains will arrive. They could integrate smart contracts (Rootstock etc.), privacy, and other nice functions - almost everything where altcoins today excel and Bitcoin does not. Then I expect many altcoins to fall into irrelevancy.

But what Bitcoin's sidechain won't be able to copy are better and more efficient consensus methods like Proof of Stake and Proof of Burn. Because sidechains can be merged-mined but there would still be a lot of energy waste and therefore, also the transaction fees would be substantial. So coins like Peercoin, Slimcoin, or Burst whose "selling point" is their efficiency and are able to offer minimal fees should not be in danger.