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.
many of us will have a bad time running around gathering up cold storage wallets to transfer to the new SC. possible privacy leaks galore, not to mention risk.
That is the point really - unnecessary risk to bitcoin, to implement 2.0 features. We should be just improving bitcoin instead. The real reason for this idea is the same reason as for everything. Profit for the side chain developers.
Has anyone a convincing argument why side chains are positive for bitcoin, rather than cryptocurrency in general? I think there is a danger we actually all collectively write the prophecy that a bitcoin 2.0 blockchain will come along and emerge in the coming years to dethrone bitcoin. It would be ironic if it was initially backed by bitcoin then destroyed it's creator like frankenstein.
I'm in hurry now but if you have a few minutes to spare just skim through episode 77 and/or 99 of "let's talk bitcoin". Somewhere Adam Back describe an hypothetical scenario where a side chains is used to migrate between from one btc version to another.