To answer your question, the goal of the WebThings platform as a whole is to improve interoperability between connected devices by making the Web of Things a unifying application layer for the Internet of Things which bridges together multiple underlying IoT protocols. The goal of the WebThings Gateway is to allow users to directly monitor and control their homes over the web, without a middleman. Building a smart home gateway just happens to be the most obvious way to achieve the latter in a way that demonstrates the benefits of the former, whilst maximising compatibility with existing smart home devices.
I agree with your observations on the traps W3C Working Groups can fall into, though it's not fair to say the Working Group tied the standard to a particular gateway project. There are multiple implementations of the Web of Things (e.g. see ThingWeb http://www.thingweb.io/) targeting a range of different use cases spanning different application domains like smart homes, smart cities and industrial applications. WebThings itself has a framework of libraries to help device makers build web things using a dozen different programming languages which have no dependency on the gateway. I acknowledge that there is a bit of a risk in implementing a smart home gateway of making the gateway itself the platform, rather than the Web of Things, and that's something I was very conscious of when designing the WebThings Gateway.
Something I think might help make the Web of Things approach more clear might be to create a general purpose Web of Things client/browser (e.g. a mobile app) which isn't tied to a gateway and can communicate directly with any web thing over the internet. Unfortunately with the current Thing Description specification this level of ad-hoc interoperability isn't possible, which is one of the reasons I created the Web Thing Protocol Community Group (https://www.w3.org/community/web-thing-protocol/), to standardise a more concrete (sub-)protocol for the Web of Things such that any WoT client can communicate with any WoT device.
It was by creating an implementation-driven defacto standard of sorts with Mozilla's Web Thing API (https://iot.mozilla.org/wot/) that Mozilla managed to drive a lot of simplifications to the W3C Thing Description standard, including dialling down the dependencies on RDF and semantic web technologies in favour of a more pragmatic default plain JSON serialisation. I hope that can continue by demonstrating traction with developers using the Web Thing Protocol in their real world IoT applications.
I acknowledge that WebThings has at times lacked an obvious direction and has not yet achieved mainstream mindshare, but I'm hopeful that things might be different outside the constraints and politics of a not-for-profit browser company.