I'm not sold on the idea that you need Web Components (and thus Polymer/React) to build or maintain a complex web app. I think that "complex" can mean a lot of different things. I know from experience that things like <canvas> and Web Workers are already showing a lot of promise by bringing multi-threaded processing to the browser for graphics- and other CPU-intensive operations. I'm also excited for WebRTC, the telephony APIs, WebGL, and other things. These things will help us build complex web apps.
When it comes to code maintainability, there is no shortage of organizational frameworks (backbone, angular, etc.), utility libraries (jQuery, underscore, etc.). These are helping me build complex apps.
What is Polymer doing for me? Allowing me to use Web Components and shadow DOM? Oh, how earth-shattering. No thanks.
I'm fine with the democratization of the VM as long as they are standards-based (and compliant). If you look at the comparison of web framework performance benchmarks, they tend to group around their VM even when compared cross-VM, or comparing frameworks built on top of other frameworks which share a base VM (e.g. vert.x is about as capable as servlet in the Java VM world; ExpressJS is about as capable as Node.js (framework) in the Google V8 world).
It seems to me that the VM is very important. One of JavaScript's latest, greatest triumphs has been its standardization. I care much more about -- for example -- Backbone.js's impact on code organization/standardization within an application than I do about its capability as a framework.
I hope that ECMAScript 6 -- while it brings awesome new functionality to the language -- will also bring with it more backwards compatibility and standardization that these frameworks currently provide (in a somewhat fragmented, yet digestible way).
And I hope the same for the democratization of the VM.
When it comes to code maintainability, there is no shortage of organizational frameworks (backbone, angular, etc.), utility libraries (jQuery, underscore, etc.). These are helping me build complex apps.
What is Polymer doing for me? Allowing me to use Web Components and shadow DOM? Oh, how earth-shattering. No thanks.