I don't get how this makes sense. People propose sidechains so that the main blockchain is not enormous. But... where are you proposing to store the sidechains? Because moving information from one place to another doesn't make it disappear.

Maybe you can say that sidechains don't need to be on the same servers as the main blockchain. But even then, to secure these sidechains you would need a similar amount of servers storing and validating these chains, and it's just the same. Not in the same space, but taking up space anyway.