“Our last major version upgrade, to MySQL 5.6, took more than a year to roll out.”
Facebook runs its own fork of MySQL, and its 5.6 branch carried more than 1,700 private patches. Some were so old that engineers had to dig through design docs to work out why they existed. They skipped 5.7 entirely. That is the real cost of forking open source. Every upstream release becomes a migration project.