Plus you're renting retail space and hiring staff but competing with companies like i.materialise, sculpteo, and so on, who can operate out of more inexpensive real estate and get the economies of scale, see Amazon. They also manage to bring together both the manufacturing and helping you find the things you wish to manufacture to begin with (which is a big problem with just setting up a 3D printer and hoping people will use it for "something").
It is a tiny market that is difficult to leverage. Far from a "surefire" anything, more likely to just accelerate their likely inevitable demise.
Let's not even talk about the huge limitations of 3D printing. It has come a long way, but realistically a lot of plastic items cannot be produced via a 3D printer because the resulting products are too fragile.