Considering Apple's support and product cycle. You are looking at a Billion devices supporting JPEG XL between now and end of next year. It might be low in percentage, but it is still a substantial number.
could you tell more/point to resources about hw-accel jpegs? does it make a difference for some sort of pipeline/batch operations or 'normal' use too? are there issues with some jpeg features being unsupported? i haven't heard almost anything about this