It'll remain voluntary, for awhile. But then , failure to chip will be considered by CPS a sign of likely neglect...Judges in custody cases will be less likely to award custody to a parent who is against implanting. And then, another kidnapping/murder of an unimplanted child, and implanting will be made mandatory.
But meanwhile, soldiers will be implanted. So will federal employees, at least those employed at "sensitive" jobs. And some parolees will be implanted. It'll start out just murderers, child molesters, and such. But just as asset forfeiture was supposed to be used only against wealthy powerful "drug lords", and is now used against so many others, often those suspected of much more minor offenses...And just as RICO was supposed to be used against the mafia, but was used against prolife activists...It is inevitable that the categories of implanted parolees will expand till all or nearly all are so chipped.
And so on. Till they have a society in which at least 40% of the legal citizenry is implanted and trackable. Then they just have to wait for the rest of us to hurry up and die.
They already do, it's called "The Cell Phone"
They will find excuses to chip everyone for some reason. Kids for their own protection, soldiers for recovery purposes, criminals for tracking purposes, etc etc etc ad nauseum.