To get a sense of what Apple's "wearables" business might look like, I think of:
1) the Watch as an iPod;
2) AirPods as an iPhone accessory; and
3) Beats as transient.
From that view, given that iPods sold about 40-50 million per year from 2006-2012, it seems plausible that the Watch could sell 40 million per year. As noted before, the Watch is cheaper than most iPods were and is far more versatile than any except the Touch.
Airpods have a meaningful cost but are not outrageously expensive relative to other truly wireless headphones. If I treat any headphones (including Beats) with a W1 (which is really the key to the AirPod) as being an "AirPod", I can imagine 10% of iPhone buyers also buying AirPods. Approximately 200m iPhones are sold each year so this is about 20m AirPods a year.
I think Beats sales (actually, sales of all expensive large headphones, of which Beats is the largest brand) are a bit of a fad so, except for those that include the W1, I assume Beats withers away over time.
Based on this and an average Watch price of $300 (this excludes accessory sales) and AirPod price of $150, we see easily $3.7b per quarter in revenues without any significant advances over what is out there today (some incremental advances over time of course, but no sharp breaks). Assuming that accessories, Beats, AppleTV and others in Apple's "other products" segment are non-zero, it seems reasonable to think there is well more than an additional $4b in revenue a year from the Watch and AirPods. Or, as one might think of them, from the S and W series chips.