That is why one reason why I suggested medical should not be among the things you can opt out of. The other big reason is that what do you with people who opt out of medical and then break a bone or get cancer or such and need medical care they can't afford.
And yes, there needs to be a backdoor option for people who change their minds later. Luckily two years of government service (or massive penalty payments) is easy to implement at any age. The penalty payments have to be really ugly though, so that the wealthy can't trivially buy their way out of service.
Jay