Sure, but my experience is that this is more in line with the client's or manager's mental model and expectations. Eg, a designer will lay out some sort of user

Sure, but my experience is that this is more in line with the client's or manager's mental model and expectations. Eg, a designer will lay out some sort of user story which typically defines "flows" from page to page. And they don't expect that the work on this new "flow" will break the existing ones. In this case sharing code can become a liability. And this sh* happened a bunch of times where other devs trying to be clever, instead of just duplicating a UI component or page and making a few changes, they put in a bunch of params to support the new use cases (a better way of course is to extract common elements). The result as predicted was a spaghetti mess to untangle. Refactoring it is like dismantling a bomb.