In that context, I considered it an advantage of polymer - that in the future, it may be a native feature of the browsers.
Do you find it likely they will abandon this idea?
(Or maybe my idea that Polymer == WebComponents is wrong?)
Polymer polyfills web components and adds some missing things on top of it, like a working templating language.
Here's all you need to know about react vs web components: https://dmitriid.com/blog/2017/03/the-broken-promise-of-web-components/
TLDR: web components are awful, react has won.