I guess that all the money Shopify makes allows them to afford weak reasoning/engineering.

Giving your users a ruby interpreter inside your infrastructure is a terrible idea. They're just one unreported bug away from disaster!

One could think of a few alternatives, all of them involving decoupling Shopify's servers from users' scripts.

It could be anything from Docker/k8s to AWS lambda to a custom DSL. I'm not saying any option is easy - proper solutions tend to require effort.