I cannot see any noteworthy economic disadvantages with sidechains.  Fewer small users, but higher transaction fees, so this part is a wash but with the significant advantage that Bitcoin need not bloat and chase out ever more of those who might wish to be infrastructure supporters.  In fact, many sidechain users would probably become infrastructure supporters for their own chosen sidechain(s), and if they can (or must) provide infrastructure support for Bitcoin itself because it is light-weight, that is a very good thing.

The main concern I have about sidechains is that Bitcoin itself has some historic baggage and disadvantages as a backing currency.  A carefully and specially designed crypto-currency could almost certainly do better.  There exists a theoretical possibility that an entire sidechains ecosystem could migrate away from Bitcoin and on to something better, although it probably would not happen without a very good reason.

That said, the 'very good reason' is most likely to be that Bitcoin has fallen victim to the kinds of attacks (mostly against fungibility) that excessive growth makes possible.  At this point one must ask philosophically, 'Is Bitcoin's demise actually that bad of a thing?  Is it not good that we have a healthy and diverse crypto-currency ecosystem which can continue to work with just a swap in functional backing store?'  Should such a swap happen I suspect it would occur by allowing people to buy in with value from the old Bitcoin blockchain, or simply automatically buy them in.  That would be the most concrete way of achieving credibility.  Alternatives include giving a pre-mine to insiders, but that would rightly collapse any possible credibility the effort might have.