It's not about optimizing for scale, it's about optimizing for velocity. I don't care if I can only get to 1K RPS. I care if my team and product can work quickly. You cannot work quickly later if you slap something together now and later realize, oh shit, we have to stop pushing features for a month so we can completely rebuild the backend and everything that depends on it.

It's the devil you know versus the devil you don't. SQL is a very well understood devil, so your plans around it will be reliable. I would argue that being able to accurately estimate future work is the most valuable business asset.