I think sidechains will be the death of altcoins mostly. But my point was actually about Bitcoin. In a round about way...
the truth is:
many alts are advanced over bitcoin in many different ways
sidechains will not change that. Sidechains will just lead to a gigantic blockchain bloat and will seal the fate of bitcoin because they will create numerous problems and only little benefit. Sidechains are the last desperate try for bitcoin to stay relevant compared to alts.
Fact is: evolution keeps going on
one of bitcoins big problems is: it's open for spam and spam in the blockchain can cause serious trouble. Even satoshi said that.
Bitcoins big problem is vulnerability to spam and the problem with blockchain bloat. Both this major problems will be made worse with sidechains.
Sidechains will create a host of problems and provide only little benefit. In no way will they end alts - they are more likely to end bitcoin if they were implemented.
All they provide is another attack vector for malicious participants.
Increased complexity will increase the risk of fatal problems.
I'd expect for sidechains to take the life out of bitcoin sooner or later.