Both are already failures - if you're realistic about it. I don't see any connection between the bad ideas of Gregory Maxwell and a few others, and Bitcoin as a system. Sidechains are one of the dumbest ideas I've seen in Bitcoin ever. The ideas was proposed 3 years ago, whitepaper written 2 years ago, and there is no functioning system and I predict there will never be.
No. Sidechains are a good idea and you're spewing out false information. If there "is no functioning system", then what is Liquid? What is Rootstock supposed to be? Lightning is quite interesting, but will be a failure, too. I will be on my beliefs by selling Bitcoin for a better alternative when it arrives. There are deep economic reasons for these ideas to be bad, but you can't expect from core developers any understanding of it. They don't even see the incentive and scaling problems as they are. "Scaling Bitcoin" is a misnomer to begin with.
It seems that everything that comes from the people which are trying to improve Bitcoin is a failure, according to the people who have no skills and/or no contributions to the ecosystem. 
No. Bitcoin dev is centralised around a small group of people, for good and bad reasons. Nobody can add just add any patch to the system. Forking blockstream/core is basically starting a new chain and project.
No, it is not centralized. If it were centralized, the main person could push any changes that they've wanted. Additionally "blockstream/core" is an very ignorant statement. Are you not capable of comprehending the difference between those two?