I think this is a baseless fear that is brought up quite often. Here me out as to why.
I'm not sure what an elephant collar costs to put on, but it isn't cheap. I'm going to guess $1k for the collar. Then, you don't exactly call the elephant like your dog and slide the collar on. You have to find the elephant, dart the elephant, put the collar on and have an entire team of trained people to do this. Let's call this another $1k per day minimum. So let's call it $2k per elephant in order to "protect" an elephant for 2-3 years.
Does anyone really and truly believe that antis are going to PAY $2k per elephant and perhaps much more in order to actually save elephants????? LOL! If you do, then please explain to me why they aren't using that money today in order to fund anti-poaching and other boots on the ground efforts already.
Anti organizations are in this for the money and want to put money in their pockets, not into wildlife. Collaring elephants would cut into their profit margin and serve no purpose. Remember, their entire business model is based on making people donate money by creating an emotional reaction based on public fear around a dwindling resource, not saving animals. A stable population actually hurts their business model.
Plus the Government of the resident country would have to permit organizations to dart every animal and not just research animals. And let's even argue that the governments allow it. How long do you think it would be until poachers cracked code and no longer even had to track elephants, but used GPS data to find the general areas elephants were in more quickly?
To be sure, we have plenty of things to worry about including hunter supported import bans, the ignorance and opinion of the general populace against elephant conservation, and and even a ban of elephant hunting in general. A fear of half a million collared elephants in Africa shouldn't be what keeps us awake at night.