I think you will find:
With dimmed cabin lights, you will have more night vision than you would with a bright cabin.
With a bright cabin, the emergency door markings and lane markings to those doors will be much less visible than they will be in a dimmed cabin. If there is a crash, who will dim the lights after the crash? The CC next to the switch panel might be incapacitated.
If you are walking along a country lane at night and one of your colleagues is using a torch - that won't destroy all your night vision unless they shine it right into your eyes. Similarly, a tightly focussed reading light in an otherwise dim cabin, or the runway approach lights at night, will not completely destroy your night vision. Whereas, a bright cabin or cockpit most certainly will. Imagine getting to a door and having to jump out onto a slide without being able to see anything and not being able to see clearly for several mins.
Re seat rows, you don't need to see clearly to count the rows - you can do it by feel. Maybe you have not done exercises in smoke filled cabin trainer rigs, but pilots and cabin crew have.
If the PA told passengers the dimming was to "enjoy the city lights", that was so as not to alarm them with the real reason.