I ran a web agency in Silicon Valley and I encountered numerous clients who's budgets were significantly hampered by having to maintain or scale poorly designed tech stacks. MediaZone was the biggest. They chose to use an in-house, proprietary ASP.net framework for their web platform. They were live streaming major sports programming back in 06/07 like Wimbledon. They had tons of customers, but their development run rate and slow speed significantly contributed to their parent company to shutting them down. It's true that most companies never get to their scale (~200 employees), but if you want to get that big, you have to plan a text stack early on that can accommodate your aspirations.

Well, how far in advance are you planning? It's easy to over-optimize when you're starting out and you end up all tangled in an overly-complex system that is over-engineered for your needs.

Everything in moderation. You can often run a single master PostgreSQL system into the millions of transactions a second without having to resort to sharding and other complexities.