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.
The value of the sidechain is derived from the market price, which ought to include both its convertibility to main chain + the side chain's utility.
&
To the extent the main chain is locked in side chains, it can be expected to reduce the liquidity of the main chain, increasing its volatility, and also the incentive to 51% it.
Awareness of the positive and negative nuances will make for better investors.