Why would you think that dogs that live in the city get any less exercise than a dog who lives in the suburbs or somewhere rural? Often cities are more walkable than the alternative.
You can't take dogs on the train. So if your apartment is in a nice, walkable place and you don't mind never walking someplace else in the city, then ok. But if you want to walk in a different neighborhood miles away that you need to get to by subway, then forget it; you can't bring your dog there.
I don't even know how you'd get your dog to the vet in a city, unless perhaps you can get an Uber driver to let you bring it in the car with you (not too sure about that; I wouldn't want to ride in an Uber/Lyft after someone had a dog in there, and other customers could very well be allergic, so services like this would be wise to ban dogs).
My dog gets significantly more exercise now than when I lived in the suburbs because there are more parks, interesting sights, etc right outside my door and I don't have to drive to them. But otherwise, sure, I'll walk. According to my fitbit I take about 25k - 30k steps a day so walking a little extra to get to a different neighborhood isn't a big deal to me.
Edit: It appears that dogs are allowed on the train/bus in DC if they are in carriers.
With a "toy" breed, that's easy. With some 80-100 pound dog, good luck with that. I don't know many people capable of lifting that much weight easily, certainly not all the single women in this area that have big dogs.
In any case they were pretty satisfied with walks 90% of the time- of course this is going to vary by breed but I absolutely advocate for getting a dog that fits your lifestyle.