[vidarh]

Yes, I can say that. The history of GSM is well known, and SIMs were introduced because having phones that were programmed specifically for one provider and that needed to be replaced or taken in to a provider to reprogram was seen by everyone including the providers themselves as an impediment to adoption.

> Unlocked phones are still relatively rare in the US so I don't agree with your second point either.

That leaves the vast majority of the world market. The US is not even the largest cellphone market any more, and haven't been for a while.

> They also trust the phone manufacturer software as they rigorously test it before it's pushed to it's subs.

Not GSM network operator has no control over what devices are on their network, just what SIMs are on it. They may or may not have control over their own subscribers, but roaming ensures that any random GSM capable device can appear on their network, E.g. I have some Chinese phone that my network operator probably haven't heard about.

> Note that I have actually worked for some major carriers and have been in discussions with VPs discussing this very issue. See my other answer further down the thread.

Unless said VPs were VPs in European carriers or manufacturers ~30 years ago, when the discussions in CELP and later ETSI led to the adoption of SIMs in the GSM standard, that is quite irrelevant.