How big percentage of customers even logged to their home router. It will be way below 10% (I would wager in lower single digit percents).

So

* manufactures open source it

* "someone" is going to maintain it, for free

* all these people are going to find non-malware infested fork

* upload custom ROM to their devices.

I just don't see it.

Automatic updates/killswitch are the only way forward.