I think the only scalability dead end you'll end up in is people-related. That is, you won't be able to scale further because it would require using patterns/solutions that balloon the complexity so much you now need a team to keep track of it, and/or it would require more money than you can manage handling on your own (e.g. you would need to turn your project into an actual business in order to fund it further - at which point you need to start hiring people, to either do the businessy bits for you, or the technical bits, whichever your preference).