What piece of non mobile/pc tech do you run firmware upgrades on more often than a router?
Good technology doesn't need to be updated all the time.
Do you receive any notification if there's ever an issue found? Is there even a centralised tracker for security issues affecting the router software?
Sometimes, things need to be updated. And consumers can't be bothered to flash a firmware image.
There could be a XSS vulnerability in the management interface.
There could be a vulnerability in any component which is internet facing.
The router is handling packets. There might be a vulnerability in that logic allowing for maliciously crafted packets (in the answer of a request).
Surely you aren't exposing that interface to the world at large, right?
Of all the devices in a house, a router should be the most important to keep up to date.
If it is not, you will have to explain exactly the vulnerability works.
Preventing DNS rebind attacks closes only one avenue.
I think saying tomato is more secure and refined than some router that updates itself constantly to secretly bait and switch the user's expectations is an understatement.