I have some doubts about some of the current wearables on the market. They're basically marketed as a fashion accessory (Apple iWatch, Fitbit) , and they don't really provide a useful benefit. They are consumer electronics, and such products are notorious for enjoying small bubbles of popularity before sales drop off. It's not something you really want to invest in.
I think the market will take off when product makers get away from the "showy fashion" trend and opt instead for a "hidden sensor" approach. For example, a wearable sensor located near the heart could continuously collect cardiac metrics and report them via Bluetooth to your cell phone, which would run machine learning algorithms that could detect a whole range of heart rhythms that require attention. The current products for cardiac patients are clunky and obsolete (and expensive) in comparison. Hidden wearable sensors are going to be the next big thing.