Starting low level and evolving to a larger scope is still unnecessary work.
It's still cheaper starting off building a playwright/calls-a-rest-api test against your web app than building a low level unit test and "evolving" it into a playwright test.
I agree that low level unit tests are faster and more appropriate and if you are surrounding complex logic with a simple and stable api (e.g. testing a parser) but it's better to work your way down to that level when it makes sense, not starting there and working your way up.
That’s not my experience. In the early stages, it’s often not clear what the interface or logic should be - even at the external behaviour level. Hence the reason tests and code evolve together. Doing that at native code level means I can focus on one thing: the domain logic. I use FastAPI plus pytest for most of these projects. The net cost of migrating a domain-only test to use the web API is small. Doing that once the underlying api has stabilised is less effort than starting with a web test.
I dont think ive ever worked on any project where they hadnt yet decided whether they wanted a command line app or a website or an android app before I started. That part is usually fixed in stone.
Sometimes lower level requirements are decided before higher level requirements.
I find that this often causes pretty bad requirements churn - when you actually get the customer to think about the UI or get them to look at one then inevitably the domain model gets adjusted in response. This is the essence of why BDD/example driven specification works.
It's still cheaper starting off building a playwright/calls-a-rest-api test against your web app than building a low level unit test and "evolving" it into a playwright test.
I agree that low level unit tests are faster and more appropriate and if you are surrounding complex logic with a simple and stable api (e.g. testing a parser) but it's better to work your way down to that level when it makes sense, not starting there and working your way up.