Conspiracy theories aside...
Some practical thoughts:
If the bracelet is not removable, you'd have guests stuck wearing it for extended periods [not something folks will be willing to do]. Assuming it is removable, then biometrics at the turnstiles will likely continue to be required for park entry. I can only surmise they will be removable.
If you're concerned about emanations from the active RFID, the power and range is unlikely to be enough to impact any medical or other devices.
With all of the different forms of tickets and types of guests, I suspect they will never entirely eliminate paper/plastic ticket media. But who knows if this is the only form of RFID they will utilize? Thus far they've been using key cards with RFID, this may be just one of many implementations being considered. Perhaps the bracelets are only for guests staying off-property and an RFID KTTW card will be used for resort guests? Maybe APs will be the only ones who get the bracelets, given the cost involved and battery life, it might make more sense that way? Until we see which available technology is going to be used by whom, it's all conjecture.
And as for "Big Brother"... Big Brother has neither the time, energy or even inclination to give a damn what the vast majority of Disney guests are up to.