Earlier quoted context omitted.
Why are the row limits so tight?! 1200 rows for the free tier, and only 5000 rows for the first paid tier! That's crazy low, given that Google Sheets gives you 200,000 cells, or 10,000 20-column rows, for free. EDIT: That was from 2009. The current limit is 2,000,000 cells per sheet: https://support.google.com/drive/answer/37603?hl=en
Probably because they are in business to make money? Customers whose first priority is cost usually aren't good customers.
They do claim a "custom number" of rows in their "contact us for pricing" plan, but the 50k number suggests that anything higher than that would have serious perf downsides with their current architecture.