They do, and obviously it is workable, but I think every big MySQL org ends up with a tool/workflow like gh-ost: https://github.com/github/gh-ost which have their own set of challenges, like not playing well with foreign keys.
Big MySQL orgs are sharded, which greatly reduces the utility of foreign key constraints anyway.
Or the Percona tools, which support stuff like online schema change.
Given unlimited VC money to burn, I could engineer a high-traffc SaaS running on a beowulf cluster of Juiceros.
Would that be a good idea? Maybe.
This is a silly argument. Facebook & Google have made massive investments into the source code of MySQL and have helped make massive operational improvements to it over the last decade. It's always glaringly obvious when people have no experience with something and then throw shade at it, like the obligatory Microsoft-bash whenever that topic comes up.