in the end, Bitcoin with larger blocks is what will win. sidechains will die and take Blockstream with it.
bitcoin will get larger blocks, just not now, "not tonight dear" ... sidechains are inevitable because they are a logical evolution of the technology (if you study self-similar network properties arising from the emergent behaviours due to self-interested individual elements you will know this), with or without blockstream (i'm picking with at this point in time).
Sidechains as embodied in Sidechains Elements is an innovative and inevitable use of the technology, I love it well done, it's what's proposed to come next that concerns me, changing the protocol to allow SPV proofs is an abuse of centralized power.