I don't think the X will be anything like airpods as far as backlog. Too many differences, mainly price and life cycle.
Once the X gets about 6 months in, as with any other phone. It's time to start weighing if it makes sense to wait for the next device.
With Airpods. They are a lot cheaper to begin with so more people are willing to buy them. Also with headphones the tech doesn't change as fast so if I didn't already have a pair. I could see myself buying the current airpods a year from now.