Yes, they can be substituted but they're definitely a staple in good German Hausmannskost. Maybe not so much in the South: Swabia subsists almost entirely on Spätzle noodles and Bavarians seem to prefer various kinds of Knödel.
But a good Rhineland Sauerbraten for example would normally be served with potatoes and a good Bauernomelette demands some crispy fried potato slices as well. Semmelknödel (but also often offered alongside potato dumplings) are a more common sight at special occasions or buffets. Many a young family's weekly rotation features spinach, fried eggs and potato mash alongside fish sticks, fried potatos with fried eggs and onions are a popular hearty lunch or late breakfast, and cooked potatos or mash are the default addition to some meat or sausage to the point a common microwave TV dinner still consists of Nuremberg sausages, sauerkraut and mash.
But a good Rhineland Sauerbraten for example would normally be served with potatoes and a good Bauernomelette demands some crispy fried potato slices as well. Semmelknödel (but also often offered alongside potato dumplings) are a more common sight at special occasions or buffets. Many a young family's weekly rotation features spinach, fried eggs and potato mash alongside fish sticks, fried potatos with fried eggs and onions are a popular hearty lunch or late breakfast, and cooked potatos or mash are the default addition to some meat or sausage to the point a common microwave TV dinner still consists of Nuremberg sausages, sauerkraut and mash.