And it absolutely might be true but do you really think the guys Blockchain, when developing sidechains, were like : "These damn altcoins! We'll show em!" 

The "major" argument is that the innovation of sidechain make most altcoins irrelevant but allow us to conserve their original intended purpose which was to innovate on top of Bitcoin.

To say "oh well network effect was taking care of that anyway so no need for sidechains" is shortsighted at best.