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.
I'm not sure what time frame you are using. At one point it was 0%, so certainly on that basis they are gaining, in some sense.
Ignoring the extreme peak in late 2013, for several months over the past year BTC was roughly $8 billion and LTC was $300-400m, which makes LTC almost 5% by itself.
I'd like to see those numbers though.
My intuition is that the opposite has happened, that alts are slowly losing ground, at least over the past year or so. I can't believe anything close to a doubling of share has happened in recent history.
EDIT: I see from the graph in a subsequent post that my intuition was correct. It isn't entirely up to date but shows alts at 4%. It's really quite silly for that to motivate anything at all about Bitcoin development. For sidechains to be a good idea they had better be a good idea independent of alts.