To Bitcoin there would be basically zero difference (between a sidechain crashing and burning and an individual losing their private keys.)

I don't know about you-all, but my interest in sidechains is nearly 100% about preserving Bitcoin and that is where my focus is.  Sidechains allow Bitcoin to freeze in it's current fairly reliable, predictable, and defensible state while not hampering the ability of it to server as the 'everything to everyone' value foundation.  That is the big appeal to me.

I'm not worried that a failure in a given sidechain will rub off on Bitcoin.  If anything I think that people will see that with sidechains,  failures are not catastrophic to Bitcoin proper.  I'll bet that there are a lot of people who are, like me, deeply concerned that Bitcoin itself is a single point of failure and efforts to grow it into the single currency for the world's masses are almost certain to trigger one failure or another and more likely than not a catastrophic one.

I'm looking forward to the new worlds that will open up as various sidechain efforts occur, and I hope to eventually use some of them for things I cannot do with Bitcoin right now, but that's pretty secondary.