Mainchain blocksize remains the same and unaffected.

There is no guarantee a big block sidechain would ever gain adoption. If it did gain adoption so what? Maybe the Bcash community would finally accept defeat and come over to BTC again use that sidechain. And it may become more profitable for miners due to increase in transaction fees.

Bold claim: If anything drivechain has the potential to stop all the politics and unite all the shitcoins to bitcoin