Comparing to Vue, I definitely get the "component" nature of its HTML/CSS/JS syntax vibes: https://codepen.io/Westbrook/pen/vYomOMW Until we have CSS/HTML modules. That syntax was one of the big draws of Polymer.
However, even if you could import the <template> as a module script, how would the platform support you insterting it somewhere? Whether that's some form of <htmlinclude> element or so for them to be adopted into the page. div.adoptedHTML.push(htmlModule) anyone?