They are so wonderful there is no need to.
Seriously, as the present units are significantly configurable by software, the only route for improvement that I see is bringing, (e.g.) USG 300 performance to the USG 50 price. Exclusive of inflation from monetary easing, this would require availability of faster processors and larger memories for less cost, less power drain, and lower thermal load. I would guess that the ability to do that with today's technology is not sufficiently advanced over 5 years ago for a new line to make sense.
A way to possibly predict this would be to consider the router software called pfSense. pfSense can be run in a wide variety of computer configurations. Ignoring its maturity, the question to ask is: Are there computer boards with chassis and Ethernet ports that have performance significantly surpassing any particular USG model that at present cost less to buy than that USG model?