Is tomato really so stable and mature that there aren't any features or improvements worthy of an official release in over a year?
No, but TeddyB is the one in charge of official releases, and he's been away since real-world events happened in his life last year.
Frankly, I wish that someone would step up to be an unofficial standin for TB, but instead we have half-a-dozen people running around helter-skelter and slinging scads of half-baked and barely-tested features into myriads of "try this!" versions.
And as nifty as the RT-N16 and RT-33(?) routers are, a plain WL-520GU works just fine for 99.99% of the people needing a router, and they don't need customizable WEB pages, super-duper VPN, webcams, network storage, etc. For example, I have 3 routers, and my 520GU is the workhorse that is doing all my wireless access and routing. My RT-N16 I use for torrent downloading and playing music — and no routing.
Freeware developers, of course, are free to work on any features they want, and are attracted to working on fun and new "sexy" and "nifty" features. But most of these are not neccesssary or useful to most users. What most users want is a stable and rock-solid router, without fragile features that are prone to locking up or rebooting the router.