What happens if one of these side chain alts becomes very successful (for example is supported by the banking system) and begins to draw a large number of bitcoins in circulation into lockdown. Will bitcoin value suddenly evaporate as all the coins sublime into the new chain in a mad rush? One can imagine some amazing pump and dump opportunities for clever side chain developers to acquire an awful lot of bitcoins through this 'two way peg' mechanism. (Early adopters again wahey!) Or to actually kill bitcoin with if a superior chain emerges.
Any successful SC will benefit the Bitcoin economy and therefore it makes no sense to suggest banking system would get behind a SC in particular.
As for the bolded statement, this is true of any alt chain, sidechain or not. I don't see how sidechains would effectively assist and benefit pump & dump opportunities. A fool and his money are soon parted, no matter the way.