One is that the future is unknown. Your goal might be to keep the product simple and scale to 1bn users, but you don't really know if it will play out like this.
Maybe you plateau at 1m users with the initial idea and pivot the product somehow. Now the app does dating, social media or specialised communications for highway construction teams.
Tradeoffs are easier to make in hindsight. Whatsapp is a good demonstration of what you can gain with good trade offs. Do less but well. Whatsapp did trade some things off though. Their web version came late, is feature poor and a little clunky. Same for a lot of features, compared to similar apps ATT. IDK what exactly can be traced to the stack, but their decisions around user identity are similar. Phone numbers only. One device at a time. They gained simplicity, but lost options.
It's easy to see the benefits in hindsight. Real time is harder. Play that game again and it might turn out different. Most apps that set out to have 1bn never get close. At this point, scalability doesn't matter and tradeoffs made for maximum scalability don't seem as wise.