Since Apple's "Activation Lock" debuted, thefts of iPhones has gone done even while thefts of Android phones has increased

More evidence that deterrence works.

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It is possible that some of the thieves have switched from iPhones to Samsung devices.  Of course, it is possible that cell phone thefts were increasing generally and that the ability to lock the iPhones caused their thefts to decline.  From a report by the San Francisco District Attorney:

. . . In New York City, thefts of iPhones fell significantly after release of Apple’s Activation Lock.  In the first five months of 2014, just after Apple introduced Activation Lock, robberies and grandlarcenies from a person involving Apple products dropped, respectively, by 19 percent and 29 percent, compared to the same time period in the previous year. This is shown in the chart below. The decrease in Apple thefts far surpassed the overall decrease in robberies (-10%) and grand larcenies from a person (-18%). Perhaps most tellingly, both robberies and grand larcenies from a person involving a Samsung smartphone, another popular device, increased by over 40 percent compared to the first five months of 2013. 

Crime data from San Francisco and London show that the introduction of Activation Lock likewise corresponded with a decline in iPhone thefts and an increase in thefts of other devices in those cities as well. As reflected in the Chart below, iPhone robberies in San Francisco declined 38 percent, while robberies of Samsung devices increased 12 percent in the six months after Activation Lock compared to the six months prior to Activation Lock. In London, Apple thefts declined by 24 percent, while Samsung thefts increased by three percent in the same time period. . . .