> it invalidates so much work that comes after it.

No, not necessarily.

I work on a large repo and new features often involve changes to 3 different services: 2 from the backend, and the frontend UI. Sending a single PR with changes to all 3 services is really not ideal: the total diff size in a feature I added recently was maybe 600+ lines, and the reviewers for frontend and backend changes are different people. The changes in the 2 backend services can be thought of as business logic on one side and interactions with external platforms on the other. The business logic can't work without integrating calls to external APIs, and the UI can't work without the business logic.

These days I open 3 separate PRs and the software only works once all 3 are merged and built. It would be great to have all of them as a single package that's still testable and reviewable as 3 distinct parts. The UI reviewer can check out the whole stacked PR and see it running locally with a functional backend, something that's not possible without a lot of manual work when we have 3 PRs.