But the franky1 point that you split the hashing power remains. This splitting makes every sidechain less secure than if you have one scaled blockchain.
Yes, not every miner will mine every chain, so not every chain will be as secure as the main chain. It's a problem. More complex designs can be used to address the issue (treechains, DAG chains etc), but I'm not sure that the dust has settled on those ideas sufficiently yet.
And who decides how many and what properties of the sidechains? You let free market decide like everyone can create one and he will compete to get haspower to get security for his sidechain (aka altcoins, and again, making average sidechain hashpower only lower) or just Bitcoin developers will have the final word?
No, sidechains will have their own interface, and anyone can design a chain to work with the interface (and Bitcoin network rules, of course)