Pictures text just fine to non-Apple users. Videos don’t go through, but that’s because of a limitation of the text protocol (and Apple is finally providing support for the extended text format that allows videos to be sent via text)¹. Claiming that iMessage should be opened up is kind of like saying that you should be forced to allow people to park their cars on your front lawn. iMessage is a non-zero-cost service that Apple provides to add value to their products. You might as well insist that Apple allow people to install iOS on a non-Apple phone.

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1. I will criticize Apple for bad UI design in that if you text a video to a non-iMessage user, it silently fails rather than alerting the sender that the video can’t be sent.