I think the SQL standard should evolve to address polymorphism like this directly though. It's incredibly common and has no direct solution afaict.

Yeah that's what I think too. This seems like a common enough issue that it should be built in. You should be able to express your inheritance schema without having to hack together triggers to keep things normalized. Any denormalization that needs to happen for performance purposes should be kept "under the hood" and used in the background when needed.

But I may be underestimating the technical challenge in accomplishing that, or maybe there isn't an easily generalizable solution that works for everyone's data model or use cases.