That accounts for significantly less than half the whole market, and half the mobile and desktop markets if you split it out. If it's also supported by other OS's and headsets and browsers that bring it significantly closer to 100%, then maybe we have something meaningful.
Well, 2 billion devices is a meaningful number of devices, no matter how you slice it.
That's entirely dependent on context. In the context of "can I use this and expect it to work most the time" it's nowhere close enough. All that matters in that respect is percentage of the whole, and at less than half, it's not really much.
As an example, we could talk about the number of connected IoT devices that are supported and up to date, and it's probably in the billions. But compared to the number of connected out of date and unsupported devices, it's likely inconsequential in comparison by metrics of total numbers, percentage of a whole, and importance (alternatively, total number of unsupported and possible exploitable devices does matter, because of what it implies about how they can be used destructively).
Is there still some licensing issues with jpeg 2000?
Every iOS and macOS device supports JPEG 2000… that should be pretty meaningful.