but sidechains are
not competitors

The same value can not be held simultaneously in the main chain and in a side chain.
But the value of the sidechain is derived from the mainchain and so the scarcity and ledger is respected.
This is a dud answer that illustrates a lack of understanding. Value is subjective one only makes value judgments, one choose to where value is greatest. It's not just scarcity that makes Bitcoin valuable, we know this because equal Altcoins with the same features can have no value.
It's the network that is created by incentive structure that creates bitcoins value. SideChain change the incentive structure.
In your ignorance you agree but you don't prove that one can create economic energy from nothing you assume that to be true. The value that is Bitcoin isn't about to die (it's FUD to say SC "are the last hope"), it's dependent on it's existing incentive structure to grow, investment in infrastructure or wealth.