Cash when you're growing so fast you're running into scalability issues is also likely to be far cheaper than cash before you have any customers to speak about.
This is more general than scalability: The more of any work you can afford to defer without hampering growth, the more of your initial capital you can spend on growing, and the more you manage to grow the cheaper it will be to raise money to build out the rest when it's time for your next investment.
The hard part is to determine which aspects you can safely defer.
Your manager was correct. People often overengineeer or overarchitect what needs to be done. In reality, you need to meet the current needs of your business. Not every single action has to account for what might happen - or what you hope will happen - if absolutely everything happens perfectly.
Obviously with some caveats. That you don't fall over at a small amount of users and you are at an early stage.