Sharia law should be everybody's concern, period. Imagine this usagi. There are kids being born into muslim communities. If sharia law is enforced among muslims, that means that if the kids grew up and renounce islam or convert to another religion, they'd be killed. If they are found to be drinking: 40 lashes. If they're found to have sex outside marriage: 100 lashes or stoned to death. When muslims say: we want to enforce it only amongst ourselves, they also mean, their kids included, who never had a choice in rejecting this barbarism.
sharia is abhorrent. And if only the islamophobes are against it, then i sure as hell hope there are more islamophobes around.
your statement is wrong on many levels.
1- Sharia law or law in general is applied by the authority not by people themselves, otherwise it is an anarchy. ( I am talking about stable Muslim Countries, not ISIS and Somalia).
2- the sharia law is not applied or only a small subset of it, is used in Muslim Countries, I guess the extreme example is Saudia Arabia, but it does not represent all the Muslim world, for example in north Africa, the law regarding marriage, inheritance is based on Sharia.
3- as far as, i am concerned the only extra laws that apply to Muslims in a a secular country is religious marriage ceremony, halal food, circumcision, and burial rituals.
so honestly your scenario is very implausible.
PS : the first time, i heard about shariaa law and taqiyaa is when i came to Australia