You do not say 'no', you propose ways to make it happen the best you can considering what you've already been asked to work on. If there is no good way then the

You do not say 'no', you propose ways to make it happen the best you can considering what you've already been asked to work on. If there is no good way then there is a management/project decision to be made (not by you). This is the exact problem. If developers can't say no then, on occasions where management or product aren't willing to compromise and do insist that a feature is added beyond the team's capacity, then the developers either have to work past healthy hours or they have to resign. That is not a reasonable point of view. Developers should not be in a position where they have to choose between overworking or leaving. Good companies accept and value everyone's input, including the developers, and trust them when they say there's a good reason not to build it. Building good software is a collaboration between everyone on the team. Management and product should not be in a position where they can demand things from developers who have to just accept what they're told. That is very unhealthy and leads directly to people burning out. This is why it's important to be able to say no.