conversely, startups that start scaling for tbs of data never make it to needing tbs of data. They burn too much energy scaling when they don't yet have a product people want yet.

Yep. I've also seen systems that were slow with <10GB data of because of bad application of patterns that were supposedly "scalable" (pulling entire tables out of the database to implement joins in application code because "nosql is faster" is not actually fast).