The business need for a user story (or the amount of work done on a user story) does depend on the ability for the development team to deliver that story. A really complex piece of work might be the best thing from the business perspective, but is it more important than multiple smaller tickets? This is where Dude's Law comes in.
Dude's Law: Value = Why / How
If something takes a lot of effort, that's effort that could be expended on something else. So if a user story is going to take a lot of effort, it's value ought to be weighed up against what alternatives could be done instead.
The inverse holds for where the how is minimal. Changes that are trivial to implement can high value even if they are not priorities from a business perspective. But a user story that's difficult to implement and of low business importance holds very little value.
Applying Dude's Law gives a way to quantify what tasks ought to be achieved in a sprint. Like everything, the code should be written such that if the project were to be shut down tomorrow, as much business value as possible should have been added until that point.