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I believe that sidechains can solve the scaling contention, precisely because sidechains do not solve scaling itself.
Let me explain.
We might define “resources needed” as X, and “production output” as Y. Then, everyone would want r = Y/X to always be as high as possible.
In Bitcoin’s case, the “resources needed” are the node costs – the infamous blocksize. The “production output” is the network’s ability – the transactions per second.
If we define “scalability” as r (ie “transactions per second” / “blocksize”), then there is no controversy about r. Everyone wants to r to be as high as possible. Everyone wants the most scalability.
However, my belief is that the “contention” (ie, the “reason everyone is upset”) is about choosing the right X. No matter the r, some people feel that X is too small (and others feel that X is too large). (This may explain why debates over scalability still persist, despite the surprising discovery of the Lightning Network, which multiplies r by a large factor.)
Sidechains are different, in that they allow people to “choose their own X”. This does not (directly) impact r, but it should eliminate most of the controversy.
(Interestingly, sidechains may also indirectly impact r, by allowing for synergy between two chains, or by exploiting [a] UTXO commitments, [b] the SC security model, and [c] the mainchain’s management of the 21 M limit, to allow sidechains to simply discard old blocks.)