SCs have the potential of giving people a false hope of it being equivalent to Bitcoin but in fact not having all of the elements of MC Bitcoin.
True someone could make a hostile sidechain. But so can people make hostile multisigs (eg freeze your coins and extort you a % of them to get them back), obviously IOU offchain we've seen no-end of offchain things be scams, thefts, or incompetence failures. Or they could make defective wallets (thats happened a number of times also).
The solution as with everything else with bitcoin is understand what you're doing and/or use software that is signed, on machines without malware, and from people who are competent. If you cant tell ask someone more competent (not you, but the general novice user). I wrote about that here:
its caveat emptor, you shouldnt put money into a chain unless there is some assurance that security & bitcoin protocol knowledgeable people have audited it. People could certify chains (like sign them - "my name is blah and I'm a security researcher with reputation and I and my buddies audited this code and its good") or wallets could etc. Its good and a feature that people can opt to use uncertified chains. You want a situation where there is real open possibility for technical innovation & competition in chain features.
You also want no central control so no chains can get black listed.
Adam