Making investments in future scalability at the cost of dev speed and performance is exactly the wrong tradeoff given that 90% of startups fail. At the start, when you have very little income, you want to allow for as much speed and flexibility as possible so that you can get to product/market fit ASAP. By the time your company gets big enough that plain MySQL/Postgres can't handle the load any longer, you will have more than enough money to afford a few experts that can help you migrate.