Can you upgrade Discourse seamlessly if there is a breaking db change?

I have no idea what you are proposing here and am having extreme issues following you.

IF we decide one day to rename the table topics to amazing-topics then old web servers running our old repo will break. Only way to keep them working under such conditions is to create a writable view during the migration that keeps backwards compat in place.

In general our migrations are backwards compat. Rarely they are not, when they are not there is a minor outage.

I am happy with you to slowly explain your technical recommendations but your one-liners here are not really helping me.