There's 2 secrets to scaling to millions of users

1. You aren’t going to have millions of users so any work you do to support it is stopping you from delivering features that will make your existing 10 clients happier.

2. Write code that can be replaced (i.e. design for change). When preparing a complex new system, discuss how it could be ripped out later, and what assumptions it has about other systems (which will restrict them when they want to change), and what assumptions the new system’s users will have of it.

Be pragmatic: it’s better to write something that’s a bit of a hassle to replace later, rather than something with a perfect interface that can have drop-in replacements deployed effortlessly, if you can get the former out the door way faster.