There are quite a few large/huge companies building stuff on Matrix - looking at the copyright statements of the code in github.com/matrix-org reveals a few. Many of them are using it in commercial stuff which hasn't launched yet - and meanwhile Matrix is still in beta, so understandably they don't publicise their activity too loudly about their activity atm.
In terms of resources: a bunch of us are paid by our dayjobs to work on Matrix fulltime, and meanwhile there's a pretty active wider community surrounding the project. However, the 'core' project of building a spec, reference server, reference+glossy web and native ios/android clients, as well as loads of bridges really is a significant amount of work, and we're going as fast as we can. https://www.openhub.net/p/matrixdotorg gives an idea of progress (although only seems to spider a few of the core projects). I wouldn't really call Matrix a 'small' project :)
In an ideal world we'd stop the world and take N months to entirely rewrite Synapse (~40KLOC of rather dense Python/Twisted) to Go/Rust/C++/Java/Node/whatever, but that's N months we don't have right now. It's not really the lack of resource, but the fact that the window is rapidly closing to provide an interoperable fabric between all of these proprietary communication apps (be they open source or commercial licensed), and we want to get a stable solution out there before it's too late and we end up in a world where the silos have won (c.f. social networks).
Hence the pragmatic decision to do an incremental migration from Synapse to Dendron, which is our next-gen homeserver written in Go (not C). Dendron is indeed mainly 'just' a proxy for now, but this is actually incredibly cool as it can act as a Matrix-aware loadbalancer across multiple Synapse backends. The horizontal scalability and HA this gives is a huge deal. Meanwhile, it gives us a way to gradually migrate endpoints from Synapse into Dendron and provide a way to replace the Synapse codebase without having to do a stop-the-world rewrite. Personally, I think this pragmatism is ftw.
In terms of verbs in URIs... we've been pragmatic rather than religious about REST naming conventions; if someone can give concrete reasons to rename endpoints before we freeze the spec, please tell us! Bugs to http://matrix.org/jira/browse/SPEC and PRs to http://github.com/matrix-org/matrix-doc please ;)
And whatever, please do come hang out on #matrix:matrix.org and #matrix-dev:matrix.org eitherway if you want to find out more and help us escape beta!