The issue was probably database migration time. I was once at a startup that had close over 1 billion+ rows in MySQL. We were approaching the `int` limit in another year or so. Many tables would need to be migrated due to foreign key constraints. Migrating one of the tables required significant downtime (6 to 8 hours, IIRC) due to slow spinning disks. Some servers didn't have enough space to rebuild the tables, so we'd want to add disks just in case. There were several servers.

A few "alter table" commands cascades to an operational PITA.