Yeah while you can survive on potatoes alone (ideally with at least something additional, like oils, herbs, veggies, etc), it isn't really a way to thrive.
Potatoes are in fact pretty bad for you when you otherwise eat well-rounded but tend to consume a lot of potatoes. That's because it's a lot of excess simple starches. Yes there are important nutrients in a potato, but you can get them elsewhere in a healthier manner.
That said, if it's the bulk of your daily diet, that means your body is subsisting on largely carbohydrates alone, and when doing that the body can tolerate it and won't necessarily gain weight. This is also true when factoring in the vast amount of effort involved with prepping and eating enough potatoes to subsist on a daily basis: you're going to be chewing and actively digesting a heck of a lot of raw material practically all day long, which means your metabolism is going to be quite high unless you are also keeping to a caloric deficit, in which case metabolic rate will drop significantly. That's what happens during a true famine period for your body, if you aren't even eating enough your body burns far fewer calories when in a resting state. Of course, that requires a prolonged caloric deficit. If you are eating enough potatoes to mostly meet your normal healthy metabolic requirements, it won't drop.
You don't want to live on carbs alone for your entire life, that is not sustainable unless you don't care about thriving. But the body is well-adapted to handle famine for short periods of time, able to subsist on largely anything made of some basic building blocks. Ideally you can find enough fats and proteins to at least stave off muscle and fat store depletion, but that is still no guarantee of long-term health.