Well, frontend-to-backend api is also unimportant and arbitrary, unless it’s designed for public use. Since for a user the page is always in sync with its server, this api is an implementation detail as much as any other internal api (e.g. between service processes or between async-threads)

Why do you think that the project separation is unavoidable in this case? E.g. some of my sites are simple express apps serving scripts and resources. I don’t have integrated build pipelines there because… well, that is exactly my concern. There seems to be no reason that we couldn’t have an ~isomorphic project with multiple entry points on multiple “ends” and a single build system for all of them.