I haven't changed the firmware on my router...
...(barring a short experiment on a disposable netgear to install openwrt) in at least a decade.
Not that the firmware hasn't been updated - Virgin Media push updates from their end. As a service they provide, it makes a lot of sense that they keep it updated (especially as it is technically their device - it is provided as part of the service). It means Joe Bloggs is freed from needing to keep track of router firmware updates and so on.
I do make sure that the passwords are all set by me, and stored in my password manager, and make sure my father at least has done the same.
OTOH, my sister's boyfriend "doesn't do updates" because they "are annoying, take time, and introduce changes" - this is on his mac, which is, as far as I can tell, running without updates from 4 years ago.
He was espousing the "if it ain't broke" idea, to which my "but it is broken, hence the fixes" fell on deaf ears.
At least my sister is a bit better at this sort of thing.