It just seems like having this debate is about repeating the same points over and over again. Case in point:
Assuming they approximate the design goal, they offer all the advantages of both Bitcoin and alt coins.
As I mentioned to davout:
Bitcoin miners can attack sidechains at very low cost to themselves, as they can earn BTC mining Bitcoin main chain WHILE doing the attack on the sidechain. Furthermore, a successful >50% attack on a sidechain is much more destructive than one on Bitcoin main, allowing the attacker to steal all of the BTC backing the sidechain. A sidechain is also likely to not have 100% of the network hashrate of Bitcoin merge-mining it, since it requires extra effort for a Bitcoin miner to setup to merge-mine another chain, so it will be easier to >50% attack.
For all of these reasons, sidechains are not as secure Bitcoin main. I recommend you read the sidechain whitepaper published by Blockstream to see the risks that sidechains face.