Ultimately, SC are what will completely destroy all altcoins and will give more and more value to Bitcoin, exactly because to the hard-pegging of the currency: what happens now is that for each new scamcoin that is launched every bitcoin holder must choose if part some of his bitcoin stash to buy some of the new coins because they may become more useful and hence more valuable of bitcoin in the future.
Oh come on. Alts in the aggregate are only 10% of Bitcoin's value, and the biggest alt has no innovative features at all. It clearly exists for some completely different reason other than that addressed by side chains and therefore is unlikely to be affected.
This obsession with alts is very unhealthy and unhelpful. Focus on the value that sidechains (or anything else) could potentially add to Bitcoin by allowing faster and safer development. There is a lot more than 10% to be gained there.
That 10% was 5% for a long time, until recent months. So alts are gaining ground. Bitcoin needs the enhanced features SC promises, but importantly, scope for scalability as well.