I personally use Polymer on a daily basis, but the lack of conversation around this library keeps me questioning its (wide-spread) adaption...
Like it or not, React won, and there is zero chance of this changing. Everyone on my team knows React, and they've never heard of Polymer. Unless Polymer is 10X better (and it is not), we're not switching.
In addition to being the de facto standard, another huge advantage of React is the ecosystem that has developed around it. React Native is amazing, and is the obvious way to go if you want to make a great mobile app (i.e., one that uses truly native UI controls and is not built on top of a slow web browser.) In addition, Redux is awesome and Polymer has nothing specific to offer there (and even if you did manage to wire Polymer up to Redux, why bother?) Finally, React is going places: React is being extended to do layout (taking over from the browser), which will finally make the browser a decent application development platform.
I'm sorry to be so blunt, but this is the way I think things really are. I am aware that (a few) others may see it differently...
And lets remember like VueJS, Angular 2.x, Aurelia that use some parts of standards that polymer builds on are also popular.
React is very popular today, like jquery was in the past. It is already visible that things are in the flux. You just project a bubble around you.
Next 5 years we will be discussing the next cool thing, the trick is - most of polymer is being implemented inside browsers already. I've seen this pattern so many times over last 20 years...