It also only lasts 5 years so you need surgery every 5 years to replace it
Hard pass on that basis alone.
The barrier of entry to theft is raised from pick-pocketing and mugging in alleys to chopping off a limb, no doubt it'll happen but I'm certain it'll be MUCH much rarer.
Yeah, in reality, the common case will be just cutting it out of victim's arm with a knife. It's much less messy than chopping off a limb, doable very quickly by a team of two people (one presenting a threat, other performing the field surgery), and doesn't require carrying around large cutting tools.
Yes, but that requires a quiet enough location or a kidnapping, both of which are far more complicated than a regular stealthy pickpocketing.
Cutting into people's flesh and taking out an implant will discourage petty/squeamish/opportunity thiefs, furthermore you'd need to know which arm the implant is in and where. I'm not sure about the exact size of the implant but cutting it out in a dark alley from a bloodied wound will certainly take more effort than grabbing a wallet from someone pocket.
More effort -> Less frequency
> furthermore you'd need to know which arm the implant is in and where
That's easy. Just swipe your smartphone over someone's arm until it beeps, then use your fingers (or phone's flashlight) to find it. Shouldn't take more than 3 seconds.