I don't doubt that side chains offer some exciting possibilities. But I am still not sure this is the right thing for bitcoin. Perhaps this if implemented into bitcoin core it will herald the end of the alt's in the future and a proliferation of side chains offering discrete services, all fully or partially backed by bitcoin.

I am not against the idea overall. But these new chains are really just alt coins initially backed to some degree by bitcoin. Perhaps that is the leverage the bitcoin 2.0 coins need to succeed in correcting some of the (perceived or real) deficiencies in bitcoin right now. Perhaps we need a static reliable bitcoin blockchain to act as a store of value, in essence like a digital gold reservoir which backs future alt digital currencies. However I suspect this is likely to be wishful thinking. I say alt currencies because these alt chains will not run on the bitcoin chain or protocol, but will simply be linked to it by this 'two way peg' system. If the last five years has shown anything, it is that 'there can be only one' is true really with bitcoin market cap being gigantic and all the rest of the alts being a tiny afterthought.

With each alt side chain presumably there will be an IPO type event where a portion of btc coins go into a black hole of being 'locked'. This will dwindle available supply of btc and instead of being sold off on exchange (ethereum) and depressing the price could actually support it. If lots of popular side chains exist the actual use of bitcoin could become reduced or even minimal. What will keep bitcoin from losing it's value? Nothing I suspect.

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.

What happens when a side chain is insecure and is 51%'d if decentralised or is simply hacked if centralised. It isn't hard to imagine a massive gox style disaster with hundreds of thousands of coins getting stolen or lost?

Interesting times.