i've heard the "solution to altcoins" argument at least a dozen times. 
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.
not those exact words. i suspect they were more subtle, like here pg 14:
5.1 Altchain experiments
The first application, already mentioned many times, is simply creating altchains with coins that
derive their scarcity and supply from Bitcoin. By using a sidechain which carries bitcoins rather
than a completely new currency, one can avoid the thorny problems of initial distribution and
market vulnerability, as well as barriers to adoption for new users, who no longer need to locate
a trustworthy marketplace or invest in mining hardware to obtain altcoin assets.