Again, the solidity of a sidechain is obtained through it's Bitcoin backing.
Until and iff all (or most?) of the BTC has moved out of Bitcoin Core to the superior consensus mechanism side chain. I don't know how likely that is or is not to happen. Just saying.
One of the only 'real' risks of sidechains is that something could come along which is so obviously superior to Bitcoin as a backing store role that it would supplant it, and sidechains would open the door for it to both do so (since they need a backing store) and because they create a much more productive planter-box.
Along a similar line of reason, Bitcoin could itself be attacked successfully whereupon sidechains could just switch to something else.
To each of these 'threats', I say "um...ok."
P.S. If one released an altcoin with such an important improvement, one might think Blockstream could just duplicate it in side chain and suck all the momentum out of the altcoin. They'd be wrong to assume that is necessarily the outcome. There is an ace up the sleeve.
As per the above, go for it! Do it quick because altcoin's glory day may be coming to an end. Or just make it a sidechain yourself from the get-go. As I say, if it is truly exceptional and/or needed you might be the new Satoshi.