Once you understand that sidechains are just that "sub-coins to Bitcoins 21M controlled supply", then it is natural to see sidechains as part of the original Sound Money plan. They simply offer a mechanism to increase functionality without a hard fork. Period
But these guys think that sidechains are somehow reinventing the altcoin threat
They don't care or wanna hear about the innovation of 1:1 two-way peg, they are busy fighting the inexistant altcoin threat
Exactly... The only feature a sidechain can add to the existing altcoin concept is decentralized exchange. If this is the only thing holding back the flood of investments into altcoins, than all of us primarily holding Bitcoin are fucked, because sidechains or not, it is coming.